Manjanggul Lava Tube Jeju Island in South Korea

Situated on Jeju Island in South Korea, Manjanggul Magma Cylinder is not at all like elsewhere on the planet. The underground magnificence is an UNESCO World Legacy Site and passage like caverns snake for around 4.5 miles (7.4km). The greatest sinkhole is home to staggering cave rock formations and stalagmites, as well as the biggest magma section on the planet.