.......whoaaaaa.

Are there entrances?

Nothing especially obvious to satellite observation. And the anomalies are all under the more dangerous sorts of biomes.

...would any Dwellin like to get paid to go look for cavern entrances.

Sounds fun! If they're getting paid well enough. These are not places one just casually wanders around.

Stoneheart is very interested in these caverns. Can her city planner blue employer introduce her to the people who know about them. She might have to quit resort city construction supervising to investigate.

They can show her the evidence they're using to reconstruct what the caverns must be like and where!

Oh. This isn't what she thought it was, it's something else. But it's very very exciting! She might want to look for entrances herself.

Presumably there's science gear that would help look for passages if she hauled it into place?

Sure... what did she think it was?

"...It touches on some things I would rather remain secret from others of my kind, so perhaps I should not answer."

...okay. Well, they can load her up with more portable versions of the equipment they found the caverns in the first place with.

She (and other hired Dwellin adventurers) will travel around using ground-penetrating RADAR and GPS trackers and so on. It takes a while to cover all that ground. A few of them die to a variety of gribblies- Active-camouflaged giant lizard, titanium-feathered Amentan sized bird of prey, relentless packs of six-limbed black coyote-things-

-There's a huge tunnel leading all the way down, here, just a little bit buried under rocks in a desert region.

...cool. What's in the tunnel?

The one who discovered it spends a while magicking an entrance, then explores a bit in, taking pictures. The walls are smooth rock, there's some fairly extensive (unrelated to the current system) writing carved periodically, and a colossal stone door about a kilometer in that the explorer says is immune to his magic. The tunnel isn't perfect- Here and there are fractures and small cracks, or large shifts, from apparent eons of plate tectonics.

The Amentans want pictures of ALL the writing, please and thank you.

Ugh, fine. It's not like he's reading it. Though he's pretty weirded out that whoever built this apparently liked writing??? ...He takes pictures.

Each section takes up a good four feet by ten feet of wall at about eye level and there are 54 sets, all unique, evenly spaced on either side. There's another inscription on the door that's just 15 characters long. There are a lot of unique characters, it's pretty clearly not an alphabet.

That won't stop the linguists from studying the shit out of it but it will certainly slow them down.

Is the door... openable?

...Not obviously? No handles or anything. This place gives him the creeps, can't the Amentans come down and poke around themselves?

They can if the wildlife aren't making a nuisance of themselves at the mouth of the cave, the greens aren't super excited about that part.

Desert's mostly about ambush predators. During daytime, at least.

Okay. They will helicopter in some people. Greys set up a perimeter, greens go down.

The slightly absurdly big tunnel awaits them, complete with indecipherable writing!

The huge door does not especially react, though with better lighting it looks fancier than before. There is some kind of slight metallic tinged colors in wavy designs like the wall art, and gemstones, on it.

They take lots of pictures. Does the door look like it can be pried open?

If they can get heavy construction machinery down here, maybe.

They will consider it! In the meantime they want to know what it's made of and check if there are signs of what tools were used and figure out how old it all is.

The wall art looks like it might have been - melted? Lasered? There's an easily-overlooked place that must have once been a campsite in an alcove made by one of the worse breaks in the tunnel that can be sent off for analysis.

They can chip off a small section of wall and door. The tunnel rock is seemingly unremarkable sandstone that somehow hasn't been subject to erosion and stalagmite or stalactite formation. The door... Looks pretty similar to the magic rocks, under microscopes.