Ice Cave
Ice caves are one of Iceland's most popular tourist attractions. It is impossible to access these caves without a trained tour guide as they can be incredibly dangerous. The natural ice waves are constantly shifting, meaning that they look different on a day-to-day basis. Ice caves can be white, blue, or even black.
Great Blue Hole
The Great Blue Hole is a giant marine sinkhole off the coast of Belize near the center of Lighthouse Reef. The hole was formed during a number of quaternary glaciation episodes when sea levels were much lower. As oceans rose, the cave flooded, causing this incredible natural phenomenon that still exists today.