A limestone cliff appears in front of you with an opening on the lower portion and you proceed through the opening. You are riding a tiny little boat with 10 other people guided by a local boatman, equipped with a handheld light up front. You enter the dark cave with the walls surrounding you, and little by little it opens up into several sections of large chambers with stalactite and stalagmite formations, some of which are utterly enormous. It is surreal to think that you are inside huge cave with a river that is 8.2 km (5.1 mi) long running through it all while being underneath a mountain. Puerto Princesa, Philippines.