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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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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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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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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Characterised by porous limestone geology and countless underground springs, the region is a wildlife-filled paradise of turquoise rivers and lakes, forested hills and massive subterranean caverns.

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Our program was looking as all aspects of underground life in Paris and in the process of researching it we also came across another extraordinary group who put the subterranean spaces to new uses.

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