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