A start of over 100-kilometer long lava tube. It got filled with mud and bat goo over thousands of years and formed a stable flat floor.
Lava caves have a different structure than limestone ones. You can see layers of basalt rock along the walls. Basalt can be more or less porous depending on the number of oxygen bubbles present in the original boiling lava.
This cave features a large tree that is rooting down to the floor since this cave gets flooded during the wet season and provides plenty of water for growth.