2,191 ± 20 metres – Krubera Cave’s difference in the altitude from the cave’s entrance to its deepest explored point, as Wikipedia mentions. Wow. Located in the Arabika Massif of the Gagrinsky Range of the Western Caucasus in Abkhazia, the Krubera Cave, or the Voronya Cave, is the deepest cave on Earth as of 2001.
Also an impressive fact is that this cave seems to be the only known cave on Earth deeper than 2,000 metres.
I must say I watched a documentary on the Krubera Cave and I was mesmerized. I realize I could never explore it myself, so this was a truly beautiful and well done video by National Geographic that I looked also online and I found it on YouTube – it has embeding disabled.
Unfortunately, there is no free photo of this cave (or at least I didn’t found one), so I can only refer you to the internet for an image search of this cave and to the another video I found on Vimeo – included below.
One thing is for sure: the Earth has many amazing places!