Mermaid Cave, or tunnels as it's often called, is a popular tourist destination. To get here, you walk over lava rock and go through an open hole. This cave is best spectated from a distance, but if you do decide to venture in, it is best to do so at low tide. To get out you have to be able to lift your own body weight, so maybe do a few pull-ups in the hotel gym to see if you are up to it!