My family has been active in caving for well over 50 years and I've been crawling around underground since I've been a kid. The past few years my brothers and I have graduated to vertical caving using ropes for traverses and rappels to explore the more harder to reach cave systems. While I dream of finding something new underground, every cave I've ever been in mostly (KY, AL, and TN) has either been surveyed or at least traveled before. I saw this on Nat Geo the other day and can't believe something this amazing was just recently discovered. It can be like another world underground and thought these pictures highlighted that well.
The first picture is the entrance to Hang Son Doong "Mountain River Cave", it was discovered in 1991 by a local, the reason it stayed unexplored for so long due to the powerful whistling noise it made from the underground river made the locals afraid of it.
It wasn't until 2009 it was made public when a British survey team finally went through it. While it doesn't hold the record for longest or deepest cave system, it does hold the record for largest (cavern size). The largest cavern in it is over 5km long, 200m high and 150m wide.
The second pic shows the other reason it stayed hidden so long, to get to the entrance was a several hundred meter rappel, which when on the top, the entrance all but disappeared into the mountain.
Anyway, enough rambling, here are the pictures.