Life-long residents of St. John, Jason Siska and Jane Conrad share the snorkeling secret of Hurricane Hole with a young local boy named Craig. As a tour guide, Jason's favorite place to take people snorkeling is Hurricane Hole. Craig has lived in St. John his whole life and, surprisingly, has never visited the remote location. Reaching the deserted part of the island, Jane, Jason, and Craig take a dive into the unique underwater environment that is unlike anywhere else in the world. The three snorkel around the undisturbed paradise that showcases colorful corals growing on the roots of mangrove trees and a variety of sea life. Craig is really excited to see a new part of the island and hopes to come back and introduce it to his friends. In Northwestern Puerto Rico, Rossano Boscarino battles one of the largest subterranean rivers in the world to explore new passageways inside an underground cave. Rossano is a tour guide and cave explorer, and today he is entering Angeles Cave with his crew to clear a log jam so he can take his tour through new areas. As they enter the dark cave, they walk through a big room with beautiful rock formations that leads them to the beginning of the underground river. Rossano explains that the most dangerous part of being in the cave is combating the river because of the unexpected flash floods. Branches and the fast water flow challenge them until they finally make their way to the log jam. He and his crew use ropes and other tools to move the heavy logs out of the way, successfully clearing out the jam and opening a new passage in the cave. On the island of Vieques, researcher Mark Martin studies the mystery behind one of the world's brightest bioluminescent bays. Mark is bringing his friend Brenda, who is also a researcher, to measure the brightness of the bay this year. This is important because around this time last year, the bay went completely dark and no one could explain why. It significantly affected tourism since the bay is the main draw of Vieques, so local scientists decided to start taking research more seriously. As night falls, Mark and Brenda head out in their boat and scour the bay for signs of bioluminescence. They instantly see fish darting around which is a really good sign, but they won't have complete confidence in the state of the bay until they measure the bioluminescence with an instrument called bathyphotometer. They place the instrument in the water, hoping for high numbers. The readings are measured right away and come out extremely high. Mark and Brenda are elated, reassured that the bay is shining as bright as ever and hopeful it continues like this.