Watch: Inside the World's Longest Sea Caves

Geologist and National Geographic grantee Nicolas Barth was studying active faults on New Zealand’s South Island when he decided to climb down some cliffs and go for a swim. That's when he discovered the longest sea cave in the world.

“Prior to this, we thought the longest sea cave in the world was about 400 meters [.25 mile] in length. Now we know that it's possible to have sea caves almost a mile long," Barth says. Other people had come across Matainaka Cave before, but it wasn't until Barth and his team systematically explored and mapped the cave that it was revealed to be a world record holder.

While finding Matainaka was a bit of happenstance, exploring an active sea cave is anything but a casual encounter. "The cave passages can be as narrow as your body, where you're sort of squeezing through and you might have to take your helmet off to be able to fit. You're holding your breath as the next wave comes in and the water goes slightly closer to the ceiling. At the back of your mind, you know the tide is coming up and you don't want to get stuck because you have a limited amount of time to get unstuck. In other cases, the passages can be big chambers up to a hundred or 150 feet wide," Barth says.

Barth’s team went on to map neighboring caves, determining that the Otago coast is home to six of the 10 longest sea caves in the world. As Barth explains, "One of the reasons that this coastline is so spectacular is because there are fractures that the caves erode along. These cause the caves to be interconnected in a really complex way that allows them to have great length. We managed to connect two caves that we thought were separate, but through this most obscure, least likely passage, crawling and squeezing sideways for quite a distance, we ended up popping up into the next cave down the coast. That's particularly exciting, when you are able to push the cave in ways that you weren't really expecting."

For Barth, the physically demanding exploration is not only exciting but also necessary for collecting critical scientific data. Since sea caves form when waves erode the coastline, the caves are full of data about coastal erosion over time. Barth's research has already shown that Matainaka is getting about three-fourths of an inch deeper each year, and his team has also documented inactive sea caves above sea level, indicating that the coast is uplifting. This type of information can reveal a lot about the rate of coastline retreat and risk for tsunamis and earthquakes. "A huge percentage of the Earth's population lives along coasts, so it's important to understand coastal erosion processes," Barth says.

The caves can also provide Barth with thousands of years’ worth of climate data because "they have some really impressive cave formations—such as stalagmites—that as they slowly drip record climate through time, just like tree rings do," he explains. "These caves have the potential for some of the best climate records on the South Island of New Zealand. That data helps us understand how New Zealand ties into the rest of the global climate."

As Barth continues to carry out research in the longest sea caves in the world, he is also advocating for their conservation. "These caves have an incredible amount of life in them. A lot of this coastline is heavily fished for shellfish and abalone. But you go in these sea caves and you can see abalone just piled on top of each other, which is totally unheard of anywhere else along this coast. It's really become a refuge, one of these last vestiges of an intact ecosystem along this coast."

No doubt, these important and mysterious caves have left a lasting impression on Barth. "What started as a fun project in my spare time turned out to be a really interesting aspect of my research these days. Some of the best days of my life were actually spent exploring these sea caves."

Nicolas Barth is an Assistant Professor at the University of California, Riverside.

Watch another exciting cave encounter as National Geographic grantee Kenny Broad gets caught in an underwater avalanche while exploring a submerged cave in the Bahamas.

And be sure to check out the entire Expedition Raw series.

PRODUCER/EDITOR: Nora Rappaport
SERIES PRODUCERS: Chris Mattle and Jennifer Shoemaker
GRAPHICS: Babak Shahbodaghloo and Chris Mattle
VIDEO: Jeff Creamer and Nicolas Barth
3D CAVE MODEL: Aaron Curtis
FEATURED IMAGE: Dave Bunnell