Several years ago, I put a downhole camera into a borehole that I suspected was drilled into a network of rotten rock, riddled with widened fracture and small caves, possibly a cavern (karst). The system was fed by surface stream leakage but sustained by what was estimated to be a very extensive hydrogeological system across the watershed (possibly more than one).
While the camera was descending, I noticed a void space in the rock possibly about a 6-8 inches across. Right after that feature, I found a white, multi-legged critter along the borehole wall who seemed not at all enthused that I was invading its space with light and movement. Shocked, I watched it retreat out of view. The camera resolution was not very high and I had no way to collect it for study. I did have the low res video, though. Later, I was able to determine it was a cave isopod, a troglobite. The closest known cave entrance was about 6 miles away. How did its ancestors get there? At that moment, I was the first person to see this creature ever. Its presence, a pleasant surprise, told me there was an actual ecosystem under there, of which I previously had no idea existed.
This creature was a troglobite, animals that live in caves their whole lifespan and their physical features reflect the lack of light in this unique environment. Geology.com has a nice feature on troglobites including a great story about the discover of an olm and how people reacted to that. Check it out.
The first known discovery of a troglobite occurred in Slovenia in the 1600s. Heavy rains flooded cave systems in the area and gushing springs carried a number of mysterious creatures to the surface. They were small flesh-colored serpent-like creatures a few inches long with legs and a flat wedge-shaped head.
The people who found these dead animals were alarmed. They thought that they had found the undeveloped offspring of subterranean dragons! A rich mythology of subterranean dragons developed from this discovery and Slovenian folk stories about them are still told today.