Located just a few miles off the coast of Puerto Vallarta, Mexico, are the uninhabited, and virtually untouched, Marieta Islands. In the early 1900s, Mexico's government took advantage of the uninhabited archipelago and performed military testing on the islands. Though the islands were formed centuries ago because of volcanic activity, it is believed that the hidden beach, and many other unexplained craters and caves on the islands are due to explosions and bombings by the government.
In the 1960s, the islands were finally labelled as a national park, meaning that they were protected against fishing, hunting, and human activity. Now, you can only snorkel and take kayaking tours of the islands – and if you're lucky, explore this hidden beach. But the trek here isn't the easiest if your scared of small spaces – you have to swim through a water tunnel that is about 50 feet long (with only a matter of feet of breathing room above the water).