If you're claustrophobic, you might not want to use full-screen mode to watch this video featuring Sweden's Dolinsjö Cave system, the largest underwater cave in the country. It was first discovered in 1979 and in the years since, a group of intrepid divers has explored deeper and deeper during annual expeditions. So far, they have reached 1.7 kilometers deep, or just over a mile. No one knows how much further the cave stretches.
To even access the cave, the explorers need to crack through thick layers of ice and load up on insulating gear. Check out the video for a beautiful look at the icy (and dangerous) underwater world.
[h/t End Clothing]