The mouth of Fingal's Cave, Staffa Island, with Mull in the distance, Inner Hebrides, Scotland, United Kingdom, Europe

Tourist framed in the frozen mouth of ice cave, with icicles formed on roof of caves at Olkhon Island as the waves freeze in winter, Lake Baikal, Irkutsk Oblast, Siberia, Russia, Eurasia

Tourist framed in the frozen mouth of ice cave, with icicles formed on roof of caves at Olkhon Island as the waves freeze in winter, Lake Baikal, Irkutsk Oblast, Siberia, Russia, Eurasia

Predjama Castle, built in mouth of cave, near Postojna, Slovenia, Europe

Fingal's Cave, the hexagonal mouth of the cave with the heather-covered slopes of Mull behind, Argyll and Bute, Scotland, United Kingdom, Europe

Aerial of La Gueule du Monstre (the Monster's Mouth) cave, Rurutu, Austral islands, French Polynesia, South Pacific, Pacific

Aerial of La Gueule du Monstre (the Monster's Mouth) cave, Rurutu, Austral islands, French Polynesia, South Pacific, Pacific

Aerial of La Gueule du Monstre (the Monster's Mouth) cave, Rurutu, Austral islands, French Polynesia, South Pacific, Pacific

Tiny figures show scale of the Deer cave looking back to mouth of the Deer cave, Mulu National Park, Sarawak, island of Borneo, Malaysia, Southeast Asia, Asia

Glass fish in cave (Parapriacanthus guentheri), large school of fish in cavern mouth, looking out to clear blue sea in background, Red Sea.

Rocky overhang, cave entrance overgrown with fern and ivy, Schweizerhaushoehle, hiking trail through deciduous forest, near Aufsess, Upper Franconia, Bavaria, Germany, Europe

Person exploring a glacial Ice Cave on the Svinafellsjokull glacier

Glacial Ice Cave, Svinafellsjokull glacier, Skaftafell National Park, Iceland

Glacial Ice Cave, Svinafellsjokull glacier, Skaftafell National Park, Iceland