The two joined Aquatica Submarines' chief pilot Erika Bergman to explore the bottom of one of the world's largest sinkholes.

By Kimberly Holland
Photo: Andrew Hounslea/Getty Images

This month, Sir Richard Branson, billionaire businessman, philanthropist, and owner of Necker Island, joined a mission to map the bottom of the Caribbean's Great Blue Hole, one of the largest sinkholes in the world.

The Blue Hole, which sits about 40 miles off the coast of Belize, is over 1000 feet in diameter and 400 feet deep. Named in 1971 by famed explorer Jacques Cousteau, the Blue Hole is a favorite with scuba divers, and it was mapped by sonar in the late 1990s.

Photo: Vincenzo Lombardo/Getty Images

However, no one had been to the bottom of the vast cavern in the middle of the UNESCO-listed Belize Barrier Reef Reserve, the second-largest coral reef in the world, until now.

The weekend of December 2, Branson, joined by ocean conservationist and film maker Fabien Cousteau, grandson of Jacques, and Aquatica Submarines' chief pilot Erika Bergman ventured further than any human has gone before aboard a remote-piloted Stingray 500 submarine. As part of the Discovery Channel's live-streaming broadcast Into the Blue Hole, the trio set out to see the floor of the vast unchartered cavern with their own eyes and answer questions about how the hole was formed.

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Previous explorations had garnered that the Blue Hole was once part of a vast above-ground series of caves and caverns. Sometime after the last Ice Age, water levels rose and flooded the structure. It eventually collapsed, forming what we see today.

Indeed, deep in the Blue Hole, limestone stalagmites and stalactites are still visible. These formations can only form on land, so their presence under all the water is an indication that this famed ocean spot was once dry.

Discovery Channel wrote in a Twitter update during the livestream that the changes visible inside the Blue Hole were evidence of sea levels that can rise "catastrophically."

After the live stream, the exploration team remained at the site for two weeks, continuing to monitor and explore the site. The data will be used to construct real-life models of the Blue Hole's geography and to help scientists understand the impact of the planet's changing terrain.

Branson, who runs Ocean Unite, a leadership initiative to help protect the ocean and bring awareness to the impacts of climate change on our planet's waters, said he joined the excursion because he wanted to bring attention to the need to protect the ocean.

You can still watch the Discovery Channel special online, and explore the depths of this vast site for yourself.