Could Underwater Caribbean Volcano Threaten US?

Ocean explorers hope the active volcano will unlock secrets to underwater earthquakes and tsunamis.
3:59 | 05/15/14

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Transcript for Could Underwater Caribbean Volcano Threaten US?
You are about to go on a journey thousand of feet down in the ocean. Here an underwater volcano could cause a massive tsunami on the east coast of the United States. And we're getting a never-before-seen look. Here's ABC's gio Benitez. Reporter: We are 6,000 feet under the caribbean sea, in the darkest corners of the volcano. The mission to save lives by better understanding the mysteries of what causes underwater earthquakes. Scientists say the menacing volcano could cause a deadly tsunami. It is the final 48 hours of 150-day voyage of discovery on board "Nautilus" with one of the most famous discoverers. Dr. Robert mallard, the man who found the "Titanic" 12,000 feet underwater. Now setting his sights on something new, the most dangerous, active undersea volcano just off American shores in the caribbean. This is the most hazardous part of our planet where plates are head-on at it. A classic example, the earthquake we had in Japan recently. The earthquake and tsunami effect that occurred in Indonesia, Indian ocean where hundreds of thousand of people died. Reporter: The conditions are so dangerous to man, Dr. Ballard and his team are relying on "Hercules" a 5,000-pound submersible to be their eyes and ears inside the volcano. Probably 3.5 million. See who else is in there? Reporter: The team has spent five months at sea. This is the season's final dive. You will see that line on the bottom of the ocean. Reporter: As "Hercules" descend I join Dr. Ballard in the ship's control room to watch the live cameras 6,000 feet underwater. Now we are reaching the top of the volcano? We are in it. Inside the crater. See it surrounded. We are inside. Oh. Reporter: On its journey, "Hercules" is doing more than taking breathtaking pictures. It's collecting samples, making maps and reaching into the volcano's plume. After hours of searching, a startling discovery. Life. That's perfect. Stop right there. What could this potentially mean? Well in this particular case, it mean that places that we thought there was very little life existing on our planet, we have just opened up a whole another area where, life seemed to be thriving. That's an alien from another planet. Reporter: Not expecting to find life, the scientists are hoping that this discovery is a small plume for better understanding these deep sea habitats. A lot of the deep sea is like a desert the we just found an oasis. Reporter: After a six-hour dive, "Hercules" is back on the "Nautilus" where it reveals I treasures. These are the mussels we collected. Seem larger than the ones we picked up days ago. Reporter: I heard you caught a record mussel. 14, he is 12. Reporter: Before this, what was the largest ever found? 13. Reporter: Dr. Ballard hopes these and future discoveries will unlock the mysteries of the monster lurking under the sea, the volcano. The key is to just be full of wonder and wanting to know about the planet and know where you find yourselves. You open your eyes and start to wonder what is over the horizon. I can't wait to know what is over there. For "Nightline," gio Benitez aboard "Nautilus." Caribbean's deadly underworld premieres.

This transcript has been automatically generated and may not be 100% accurate.

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