TEARS OF THE TURTLE is the deepest known cave in the lower 48. It was discovered in the Bob Marshall Wilderness of Montana in 2014. I don't have any inclination to go exploring around so far underground (I much prefer to be on the mountain), but I do find these caves quite interesting! :)
"Ballensky bested his own record when he returned to the Bob and descended to the deepest known point of the Tears of the Turtle cave, which is 1,629 feet below the surface, making it the deepest cave in the continental United States."
See pics and read more in the Flathead Beacon.