Los Hervideros, lava caves on the rugged Atlantic coast, El Golfo, Yaiza, Lanzarote, Las Palmas Province, Canary Islands, Spain, Atlantic, Europe

Lava Tube, Santa Cruz, Galapagos, UNESCO World Heritage Site, Ecuador, South America

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

Columnar basalt lava with Fingal's Cave cut into it by sea, Staffa, off west coast of Mull, Inner Hebrides, Scotland, United Kingdom, Europe

Archway Cave, segment of very large lava cave, Undara Lava Tubes National Park, Queensland, Australia, Pacific

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

Diver at Caves of Lava Tubes, Cathedrals of Lanai, Maui, Hawaii, USA

Diver at Caves of Lava Tubes, Cathedrals of Lanai, Maui, Hawaii, USA

Spiral staircase leading downward into a lava cave, Casa Museo del Campesino farm museum, Mozaga, Lanzarote, Canary Islands, Spain, Europe

Lava cave , Sao Vicente, Madeira, Portugal

Entrance to Thurston Lava Tube, Lava tube, Hawai'i Volcanoes National Park, Big Island, Hawaii

Water reflections in the cave Cueva de los Verdes, illuminations of the cave system of a lava tube designed by Cesar Manrique, Lanzarote, Canary Islands, Spain, Europe

Divers are framed by the twin openings at a dive site known as Skull Cave off the Kona coast of the Big Island, Hawaii, United States of America

Diver at one of the many openings around the dive site called First Cathedral off the island of Lanai, Hawaii, United States of America

Diver at the entrance to First Cathedral off the island of Lanai, Hawaii, United States of America

Diver outside of the dive site called First Cathedral off the island of Lanai, Hawaii, United States of America

Diver inside a cavern known as First Cathedral off the island of Lanai, Hawaii, United States of America

Diver at Caves of Lava Tubes, Cathedrals of Lanai, Maui, Hawaii, USA

Diver at Caves of Lava Tubes, Cathedrals of Lanai, Maui, Hawaii, USA

Diver at Caves of Lava Tubes, Cathedrals of Lanai, Maui, Hawaii, USA