Watch man and ticket office at Bimmah Sinkhole (doline), near Tiwi, Sultanate of Oman

Bimmah Sinkhole (doline), near Tiwi, Sultanate of Oman, Arabian Peninsula

Bimmah Sinkhole (doline), near Tiwi, Sultanate of Oman, Arabian Peninsula

Sinkhole Plain, polygonal doline karst, near Mammoth Cave, Kentucky, United States of America, North America

Tokyo, Japan

Modro Jezero, Blue Lake, Imotski, County of Split-Dalmatia, Croatia, Europe

A cave explorer stands on rimstone formations in Hang Son Doong, Vietnam.

A cave explorer on a speleothem at the first doline, or skylight, in Hang Son Doong while two other explorers illuminate a large passage leading to the second doline 1.5 kilometers away.

A cave explorer illuminates a large passage looking out towards the second doline in Hang Son Doong, Vietnam.

A caver is dwarfed by massive stalagmites in Hang Son Doong. Light can be seen coming in in the distance through the frist doline which is over 3 km away. Hang Son Doong is located in Phong Nha Ke Bang National Park, Vietnam.