The ceiling's very low and the melted water runs too fast. The walls are slippery and light fades quickly as you go farther away from the entrance.

But the cave's ceiling is made of precious crystals and one look at it is enough for you to remember that no matter the effort, it is all worth it. So you push forward.

You're in the Sapphire Cave, in the heart of Iceland's biggest glacier, Vatnajokull.