Because most of the peninsula sits on a limestone shelf, there are no surface rivers, and rainwater quickly seeps underground where it gathers and forms subterranean bodies of water known as cenotes.

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This spider's web of narrow tunnels, complete with coffin-sized cavities, opens out into dark subterranean churches - a most evocative way to end an underground tour of Rome.

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Ajit Chambers is a former City of London banker who quit finance to set up the Old London Underground Company in 2009, which is leasing the subterranean land from the city.

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Another of the terminal's great subterranean secrets is M42 -- a hidden basement and the deepest accessible underground point in Manhattan.

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