Caves can be found on every continent, within glaciers, and even, as seen here, under the oceans. Most simply, they are defined as a "natural underground space large enough for a human to enter."
Typically, a true cave has at least one portion that cannot be reached by natural light, though in common usage, a "cave" can include any opening into the earth.
Image credit: Kim Westerskov/Getty Images
Get up close and personal with the natural world with these amazing photos.
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