Changing Planet

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.”

Nicolas Barth’s team also mapped Great Fangs Cave, located on Otago’s coast. Photo by Nicolas Barth.

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.

Nicolas Barth squeezes through a narrow passage while exploring a sea cave on Otago Coast.
Nicolas Barth squeezes through a narrow passage while exploring a sea cave. Photo by Jeff Creamer.

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.


SERIES PRODUCERS: Chris Mattle and Jennifer Shoemaker

GRAPHICS: Babak Shahbodaghloo and Chris Mattle

VIDEO: Jeff Creamer and Nicolas Barth

3D CAVE MODEL: Aaron Curtis


Nora Rappaport is a producer and editor on National Geographic's Science and Exploration Media team. She produces content that highlights the awe-inspiring work of National Geographic explorers around the globe. When not working with her colleagues to inspire people to care about the planet, Nora can be found hanging out with any number of dogs.
  • Herbert Mahler

    This is an awesome story about Nicolas Barth’s Matainakas cave exploration and mapping and it is so important for our
    better understanding of the climate changes,coastal erosion
    and erdquake and tsunami data. great job Nicolas Barth

  • Brian

    This is incredibly exciting research! Another element to the huge mystery of life underwater and along our coastlines

  • Jeff Mather

    More great work Nat Geo! The photos are amazing. Thank you!

  • Julie Capps

    Love the video on the Longest Sea Cave. Amazing!

  • Kelly Huckaby

    What an amazing experience and important discovery for Nicolas and his team. It is always inspiring to see people out in this vast world making new discoveries through their own curiosity or through the work of Nat Geo! Well done!!

  • Jim Kulik

    This is a remarkable finding that has both basic and applied value. Prof. Barth should be commended for his discovery and for the courage it took to bring this information to the world. Well done!

  • Michael Pazzani

    Amazing. Congratulations Dr.Barth. Nice photos and great story.

  • Dick Thompson

    This is a very interesting article about a not so familiar topic. It is enlightening about sea caves being an important source for study and research. It’s lucky for us people like Nicolas has the nerve to explore places like Matainaka. Good work Nicolas

  • Irmgard Thompson

    What an amazing Job Nicolas, the photography and your narration are fascinating. I am looking forward for the next
    Article of your well rewarding outstanding work.

  • Maryam

    Excellent work Nicholas Barth and Nat Geo!

  • Thomas Bowie

    Great article on Caves, just excellent.
    But better you do the diving than me.

    Look forward to your next adventure.

  • Thomas Bowie

    What a wonderful article Nicolas, your parents and grandparents must be very proud of you.

    Your discovery lets us know there are many things left to discover is our undersea world. Continue on with your research projects as they will be important to our world.

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