It's possible, but that doesn't necessarily mean that drop will lead down to Hollow Earth. If you think about the first time we enter Deep Caverns, in approximately the south-west corner of Hollow Earth, that would put the entire facility somewhere between the Institute and Core, straight down.
So that abyss you see from the edge of the creeper's nest might be somewhere over the eastern side of Deep Caverns, or perhaps an unseen section of the caverns. There are lots of destroyed archways scattered around Deep Caverns, hinting at places we are told of in records but can't physically visit anymore, such as Tartarus. Who knows? That drop in the Black Sea could even connect up to the Faceless City.
I've tried to think of a way to accurately estimate depth in UnderRail, and I haven't been able to find a good one. The ambiguity in the length of the elevator rides is the main issue. Relating two places that aren't on the same elevation seems to be guesswork.