After viewing the pictures of  the exploration of Kubera cave, I asked myself why is there a depression in the sand at the bottom of the cave? Certainly if this is the bottom of a hole, where is the water that must seep to this level of a hole in the ground?

 This bottom has to be a sandy, rocky filter that allows the water to drain even deeper.

 Removing the plug could be very difficult, or not. But drilling an exploratory hole for a camera could prove very exciting results.