Sirena Cave was once made into a tourist place. It has since fallen into total disrepair. For anyone on island who happens upon this cave, I don't recommend entering: I have heard that there is some kind of natural poison gas somehow generated by natural decomposition of stuff inside it. I have also had friends who descended into it and became ill, so that makes what I heard about the gas very convincing. I believe that the owner actually had a forced-fan ventilation system when he allowed tourists in.
If you are curious about the story of Sirena, see the following: http://www.optimimagery.com/TRAVEL2012/OCTOBER-2012-GUAM/26054040_crwfZL