yes there could be a void area underneath where the water goes...
there was a japanese outpost in that place during the war, that according to an old folk, people passing in that area should show proper courtesy to the Japanese soldiers on duty or else they do something terrible punishment to the Filipinos who dont show respect to them.
For me, that outpost/checkpoint is just a bluff to create fear to the local folks, and prevent them from passing in that area, the Japanese might had something in that place so they are securing the area.