This crack in the rock was taken over by bats. Thousands of bats. Lovely. Also note that this is all volcanic rock deposits. In the center of the picture just above the waterline, you can see a block of stratified tuff that was ripped up and carried in a pyroclastic flow and eventually deposited here were the whole mass of rock solidified. Subsequently, wave action has eroded the cliff and carved these little overhangs. The bat cave itself is probably just a small fault.
Not too far from here, we went for another snorkel which was really neat. I saw a bunch of fish and an octopus that blended in so well that I couldn't see it even when I knew exactly where it was. It was only when I got close (~3 m) and it moved that I was able to see it.