The colors inside the magma chamber are attributed to microbes, sulfur-rich gases and rocks tumbling off the walls. (Image credit: Pedro Carrilho (left and right) and KKKvintage (middle) via Getty Images) QUICK FACTS Name: Thríhnúkagígur, or Three Peaks Crater Location: Southwest Iceland Coordinates: 63.9984, -21.6989 Why it's incredible: The volcano hosts the world's only magma chamber that's accessible to humans. Thríhnúkagígur is a dormant volcano near Reykjavík whose last eruption 4,500 years ago left a hollow magma chamber with bronze-and-indigo-colored walls. It's the only place in the world where people can climb into a volcano's plumbing system and explore the cavern that held sizzling molten rock before an eruption. Since 2012, visitors have been allowed to enter Thríhnúkagígur's magma chamber via an open cable elevator that was built in 2010 for scientists. The chamber is about 700 feet (210 meters) deep, or more than double the height of the Statue of Liberty with the pedestal.…