Monday, February 27, 2023

Beach without sunshine: The Cave of the Seals

Imagine summer. Imagine a nice beach in a southern country, like Greece. Way too hot! How about a beach in a cave, so that you don't have to get sun cream out? This, and so much more you can get in the Cave of the Seals, north of Corinth and Loutraki and on the Perachora peninsula. An hour from Athens.

The most remarkable feature, of course, is the stone arch above the waves.

Unfortunately, when I was here in mid-February, it was stormy winds, and obviously the water was also cold. The waves were way too big for me to safely attempt to cross to the cave side.

The coordinates for this place are N 38.07414 E 22.98441.

Depending on how sporty you feel about driving off-roady roads, you can park either here (easier) or here (closer, more remote, difficult road).

Some more information about the Cave of the Seals can be found here and here.

The descent to the beach and cave is steep and mildly challenging. A rope is provided:


Pictures from the bottom:

The cave itself:

This article has also appeared at TGR. Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2023 by Jari Arkko. All rights reserved.

Sunday, February 26, 2023

Tower and Cave in Alíartos

A random discovery while driving through Greece's small mountain towns. A massive hole by the road, with a tower on of the rock hill. This was so sudden that I didn't even think of registering the location or stopping. And I was in a rush to get to the ski area anyway.

But I hoped that I would find it on the way back. Or from the Internet, and could ensure that I take the same path. But it was difficult to find, nothing came up with regards to caves. And when I saw it on the drive-by I thought it was a church. Given that the cave was open towards my inbound direction, I was worried that I would miss it on the drive back, even if I drove the same way.

But, fortunately my navigator did take the same path back, and I was alerted by the tower that now we might be approaching this location. 

For future reference and other visitors, the coordinates are N 38.368001 E 23.120891.

And it was great. The tower is the Tower of Alíartos, a medieval Byzantine defense tower. Currently standing 15 meters tall, but may have been higher. Here are pictures from the tower:

There are several caves in the rocky hill that the tower sits on. The largest one is by the road, with a large entrance opening, a large hole in the roof as well, and a smaller "window" to the side. The cave is about 25 meters. At the end it narrows down to a small crawlspace, which I did not crawl, but I'm not sure if it ended or led to the next door small cave hole in the hill face.

Why didn't I crawl? Because as a roadside attraction, the cave seems to attract people who need a bathroom break... toiler paper and "droppings" all over the place :-) including stuck in my shoes... sheesh

Here's a video of the cave, a fly through of the 3D model I recorded. The fly-through is an animation through the model in Blender, and it took 180 CPU hours to generate the video:

You can also rotate the model freely on your own browser window by here. Or download the model in GLB/GLTF, Blender, or STL format.

Below you can see the Cave Outliner-generated cave map (there's also a more high-resolution PDF version):

More pictures from the cave:

Somebody needed to use the toilet?

And here's the tower:

The article has also appeared in TGR. Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2023 by Jari Arkko. All rights reserved.

3D-printing 3D-scans of caves


Can we copy a cave, by scanning it with an iPhone lidar, then printing a model of the cave using a 3D printer? The short answers is yes, but there's a lot of complexities, and it is not as easy as one might initially think.

I visited Tero yesterday and we got into trying to use his 3D printer to print 3D cave models. Tero has a lot of experience with modeling and printing, but there was one difference to what he's usually printed: the models.

To begin with, one normally prints solid objects, let's say a toy that is an object that can be lifted and held. There's a surface and inside the surface is some material (plastic) that's inside the object. 3D models fed to 3D printers need to have manifold geometry. As one source puts it, "non-manifold objects have geometry that cannot exist in the real world". For instance, shapes that have no volume, objects that have only one side, etc. Read the article in the link for more details.

The problem is that cave scans -- at least the ones that I've created using the Polycam app -- really are not about objects, they are about space. More specifically, they are about surface forms, without real knowledge of what's on the other side of the surface. As a result, they are objects that have no volume. One way to think about this is that they represent a shape and texture of an infinitely thin film around the cave.

Trying to feed such models to a slicing application or a 3D printer results in an error: the geometry is not closed.

Intuitively, one may think of caves as objects if one perceives the space in the cave as the object. But this intuitive thinking also leads us to some problems. For instance, if the space in the cave is the object, would the object distinguish between the border between the cave and its wall, vs. the border and outside world at the entrance of the cave in some different way? In speleology, a "drip line" is defined as a line on the ground at a cave entrance formed by drips from the rock above (Jennings). One could of course take this definition as the surface between the cave's outside and inside. Still, it feels at least to me intuitively odd to represent a hole, an entrance, as solid material.

Anyway, from a practical purpose, what can we do to print these weird models as objects? Or manipulate them with traditional CAD and 3D modeling tools that are often based on operations such as join or intersection?

Tero came up with the idea that in Blender one can "extrude" faces. A "face" is a triangle shaped fragment of a model with a "normal" that points to the outer side of the face. In a traditional model of an object, the normal would be pointing away from the insides of the object. E.g., a face on the surface of a ball would point away from ball's center. In my cave models, the normals are pointing from the cave wall faces inwards to the cave.

Tero put the Blender in editing mode for the cave object, selected all faces, and then extruded towards the outer direction (against the normal), so that the model would grow from an infinitely thin surface to something that has some substance. For our first iteration of this the grown cover turned to be too thin, less than the width of the printer's smallest print detail. More seriously, the slicing software left gaps between individual strands of printed plastic, making the cave have holes. The structure was neither pretty or able to hold itself properly.

Our next iteration increased the amount of extrusion. But it too failed, as the cave was now built upwards, and it fell down in the middle of the printing.

The third iteration added a bottom that would glue the object to the platform while being printed.

This worked well from the point of view of the printing. Here's the first (left) and third (right) iteration.

You can tell that in this small model, there are some imperfections in the surfaces. And the model itself is missing a part of the roof above the entrance -- a problem caused by iPhone's limited lidar range (5 meters).

To some extent we should be able to deal with this with better input models, by printing larger objects, and using Blender's tools to match the granularity of the model to the capabilities of the printer. And the right choice of support materials under the model being printed, right 

But is this great and all is fine? It is great in that for the first time I've managed to print something that I scanned, a real-life cave. But if we look more closely, there are issues.

To begin with, there's a serious issue with extrusion as provided by Blender's tools. They extrude every face, along (or against) the normal of each face. Here's a picture of the normals, as shown in the Blender's manuals:

In a complex shape like a cave there are places where extrusion against the normal will hit something else. Imagine a extruding the faces making up a stalactite hanging from the roof of a cave, as in here:

A straightforward extrusion hits multiple issues. First and most serious, in places where the form of the cave is very near the backside of a face -- as in the tip of a stalactite -- the extrusion will actually come out of the other face and show up as an error in the inside surface of the cave. Even if we never wanted to change anything in the inside.

So this is where we are now. Stay tuned, we're thinking about possible ways or algorithms to resolve this in a better way.

Another problem that we'd like to address is for a different kind of a print. So far, we've printed cave models where we'd like to peek inside. This is only practical for smaller caves that we can easily see inside from the entrance. For a large and more complex cave another type of a model might be preferable, such as a solid representation of the cave space as plastic. You could then see the branching cave passages and the overall makeup of the cave easily, but not see inside. For instance, I'd love to be able to some day print a model of the Lummelunda cave in Sweden:

This could potentially be done in a same extrusion fashion but choosing to extrude along the normal towards the insides of the cave. But it too would hit similar issues in complex cave geometry. Another alternative is perhaps enclosing the entrance(s) with a drip line cover in the model, and then considering the cave model as a 3D closed object.

A third problem is that in addition to the main, legitimate holes such as an entrance, scanned models tend to have smaller holes or missing parts of the model, e.g., behind rocks or small turning passages that one can't easily reach to scan them fully. These need to be blocked for any these approaches to work well.

Oh, and where can you get some 3D cave models, if you want to test? I have some on my web page, here. They are freely downloadable.

This article has also appeared at TGR. Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2023 by Jari Arkko. All rights reserved. The normals in a donut picture is from the Blender manual, copyright Blender project.

Saturday, February 25, 2023

Cave Lakes: World's Highest Cave Pools in Greece


A loved child has many names: Kastria Cave, Cave Lakes, Troupisio, and, of course, Σπήλαιο των Λιμνών. I'm in Peloponnese, few hours west of Athens. And in the village of Kastria, in the municipal region of Klitoria. 

The Kastria Cave is 1980-meter long cave running in three levels, an ancient underground river. It was officially discovered only in 1964, and is currently a tourist cave with regularly scheduled tours. But signs of human use in the lowest level of the cave, near the original entrance date back to the Neolithic Age. It is believed though that the early humans did not reach the upper levels of the cave, as they would have required rope ascension techniques. More information about the cave can be found from the English (shorter) wikipedia article and the Greek wikipedia article.

What's remarkable in the cave is that it still continues to have some water, and the cave forms so called cave pools or rimstone dams. According the cave owners, this cave may have the highest such dams in any known cave-in the world, up to 6 meters high. That's definitely very large. A remarkable sight!

There's more water in the winter than in the summer, though now this year (2023) it has been a dry autumn so water levels were low. 

And the cave is an exceptionally good shape, there's a metal walking path through 500 meters inside the cave, but otherwise the cave seems unbroken, untouched, and only moderately lit. Very nice! And particularly important for the large number of bats that live in the cave.

The part of the cave that you'll walk through has primarily large halls of the second level of the cave, up to 36 meters high.

The cave is indeed open for tourists. Due to the concern of disturbing the ecology of the cave, tours are limited in duration to 30 minutes, which feels slightly fast, but if it helps the bats I'm ok with it :-) 

What's more disturbing though, and in my opinion unfounded, is that photography is strictly prohibited. Not following my usual behavior, I actually stuck to the rules this time. However, I have gotten some pictures from outside the cave, where we can see the original entrance right by the road.

The tour costs 9 euros per person, and they run I think until 16:30 every day. Here's the website for the cave tours. Much recommended, you have to see it for yourselves!

The original entrance is shown at the top photo, as well as in the two below photos. Note that I did not enter the entrance, just took photos from outside the gate:

From the insides there's nothing to show except the path leading to the entrance, and a photo of a postcard I purchased. (Copyright of course by the original photographer).

Here are some photos of newspaper clippings with basic maps of the cave. Copyright the original magazines of course. Note that these clippings were not inside the cave, they were in the entrance building:

And a nice sunset on the way back:

This article has also appeared at TGR. Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2023 by Jari Arkko, except as otherwise indicated above for the newspaper clippings and photos of postcards. All rights reserved.

Thursday, February 23, 2023

Kalemegdan Park Castle

There's a castle at the Kalemegdan Park in Belgrade, the Belgrade Fortress.

There's free access to the castle grounds, much recommended particularly on an evening with sunset views.


Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2022 by Jari Arkko. All rights reserved.

Wednesday, February 22, 2023

I don't know what this is, but it looks somehow familiar...


Greetings from your sewer inspectors! This adventure in Ancient Corinth, Greece, started in a very promising way: why not visit a cave whose name is Σπηλιά «αιδοίο» της Αφροδίτης, or, Aphrodite's you-know-what? And I have to say, it does look very familiar, in fact it looks exactly like the Finnish Högberget cave. See below:

Internet sources (here and here) seemed to confirm the story about a cave, a spring, and an ancient place of bathing.

Here's a video of going through the cave, constructed from a 3D model:

There's also a map, also automatically constructed from the 3D model using the Cave Outliner. The main map picture is below, and the full PDF map is here. And if you want, you can rotate the 3D model on your screen by clicking on this link.

Anyway, back to caves... or sewers. I really don't know what the origins of this cave are, I suppose it could be a rather symmetrical crack that has overgrown with calcite formation over a long time. Or, it could be an ancient sewer tunnel cut by man. I didn't know what the answer was, but upon reaching the end of the tunnel, and seeing straight tunnels leading up towards the housing sitting above the cliff, I'm now guessing it is actually a sewer, again overgrown with calcite formation over centuries.

More pictures:

In front of the tunnel part of the cave there's a shelter cave (a cliff extending above you):

Stairs from nearby road lead down to the small valley the cave is in:

The cave's coordinates are N 37.91062 E 22.87992.

This article has also appeared at TGR. Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2023 by Jari Arkko. All rights reserved.

Tuesday, February 21, 2023

Cold swim in Ancient Corinth

Brr. A bit cold, maybe 5 degrees. But no matter. The views are great! And I've never swam in Greece before, so now I have to. 

On my ski trip I was staying at a new luxury hotel, the Oliveus Euphoria Resort. It was in a very remote location in Ancient Corinth. So remote and new that Google neither new about the place or the roads leading to it. It was a hassle to get there, given this.

But maybe worth it, given the olive tree garden, views, and the nice common kitchen area.

The pool was a year-around pool, but without heating. And in general, I was surprised how cold it was in Greece in the wintertime. Snow doesn't even seem to be such an exception, peaks around the Corinth area all carried snow.

I only managed to get breakfast on one day though, in winter the place is not busy, so the staff seemed to forget my existence. No breakfast materials or exit gates open... oh well, I got to stay there at an incredibly affordable price, and I liked the room. And the swim, obviously! Very refreshing.


View from my room:

Kitchen area:

For more sauna and swimming stories, check out and websites! And of course the Planetcaver, and Planetskier blogs for other stories in Blogspot and TGR! The photos and text (c) 2023 by Jari Arkko. All rights reserved. I never take photos of other saunagoers or swimmers and visit when there is simply no one else or the facility has been closed or booked only for me. Or like in this case, when I was alone in the entire facility, and no one in their right mind came even near the almost frozen pool :-)