Photos show what cave divers discovered when they swam inside an iceberg the size of Jamaica. Today, it's gone.

• In 2000, the largest iceberg ever recorded — Iceberg B-15 — broke away from the Ross Ice Shelf in Antarctica. 
• Underwater explorer and photographer Jill Heinerth dove inside B-15 and snapped photos of this hidden world.
• Today, nearly the entire Jamaica-sized iceberg has melted away. Heinerth's photos offer the first and last look inside it. 

In 2000, when an iceberg the size of Jamaica cleaved away from the Ross Ice Sheet in Antarctica, Jill Heinerth saw an opportunity to make history.

Heinerth is a professional underwater explorer, cave diver, and photographer. She's been diving in the most remote parts of Earth's oceans for 35 years. Her trip inside B-15 marked the first time anyone ever dove beneath an iceberg.

It's not uncommon for Heinerth to be among the first humans to venture inside these hidden places. But with B-15, she was also among the last.

When B-15 broke free, it was among the largest moving objects on the planet. Today, it's lost 99% of its size and only one piece remains — a chunk measuring roughly 40 square miles, smaller than Disney World.

Luckily, Heinerth and her dive team seized the opportunity to explore and photograph the inside of B-15 while it was still a behemoth 23 years ago. And they risked their lives to do it.

But for Heinerth, the work is worth the risk. "For me, diving in these icy environments is almost like documenting an endangered species," she said.

Through her underwater photography, she brings the story of these disappearing ice caves to the surface, aiding scientific discovery and raising awareness about the rapid progression of climate change.

Swimming in uncharted waters

To reach B15, Heinerth and her dive team sailed for 12 days across the tumultuous Southern Ocean, weathering 60-foot swells and knocking ice off the boat with baseball bats. When they finally reached their destination, even more dangers lay ahead.

"During the trip, we had many close calls," Heinerth said. She tells the full story of her three death-defying dives in a 2019 WBUR article.

On the initial dive, the team faced its first brush with danger. After descending through a long vertical crevasse in B15 all the way to the sea floor, 130 feet down, Heinerth spotted the entrance to a cave leading into the iceberg.

Once inside, Heinerth described it as "this dynamic environment that's beautiful. You see how the sea has sculpted the ice, like there's these great scallops that are carved by the hand of the sea," she told BI.

But suddenly, they heard a deep groaning sound.

Massive chunks of ice had fallen into the cave entrance, blocking their way out. Luckily, they found a way through and escaped with their lives, ready to return the next day.

During their second dive inside B15, they got caught in a powerful current sucking them deeper inside the iceberg. They couldn't fight it and instead rode the current through until eventually it took them to another exit on a completely different side of the iceberg.

But even that wouldn't prevent them from taking a third and final visit, when they faced the most dangerous dive of all. On that day, the powerful current hit again. This time, there was no backdoor to escape through, and the flow was so intense that even when they fought their way back to the cave entrance, they couldn't rise back up through the crevasse.

"On our very last dive in this environment, we were pinned down from the currents inside the ice and having difficulty getting out," she told BI. "Our one-hour dive turned into a three-hour flight for our lives."

What saved their lives was when Heinerth remembered the burrows that fish make in the crevasse walls. Using these holes as climbing holds, she and the team slowly climbed their way against the current and eventually made it to the surface.

Then, just hours after they resurfaced, "The entire piece of ice that we'd just been inside of literally exploded and turned into a sea of slush ice as far as the eye could see," Heinerth said.

"I was just standing there, gobsmacked on the ship's rail. I realized that if we had been in the water, we'd be dead," she wrote for WBUR.

Documenting a disappearing world

Between moments spent fighting for her life, Heinerth managed to snap photos of B15's inner world. As a citizen scientist, she hopes that documenting these rapidly changing environments helps researchers better understand them.

"We are now living in a time of very clear existential threats — a time when we need an army of citizen scientists that can provide reflections, anecdotal evidence, but also data gathering streams," she said.

Over the last two decades, Antarctica has lost an average of 150 billion metric tons of ice per year, NASA reports. This rapid melting means iconic arctic species like humpback whales and emperor penguins are losing critical resources and habitat.

It also means sea levels are rising. "When [B15] broke off — when you have ice that moves from land to sea — that's going to change sea levels globally," Heinerth said. "And that's the thing we need to be concerned about."

Exploring the below-surface geomorphology (the shape and structure) of icebergs like B15 can help climatologists understand how quickly they're disappearing, she said. As they melt, water moves downward through cracks in the iceberg, carving out new caves and crevasses for intrepid folks like Heinerth to investigate.

Through her dive photography, she hopes to spread awareness about how quickly these hidden ice environments are changing.

"We're at the point where we need to make some climate interventions, and that is going to require political will. And it's going to require a global citizenry to be relatively educated about what we face in the very near future," Heinerth said.

Photos courtesy of Jill Heinerth. Learn more about her work at: www.IntoThePlanet.com