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There are numerous sites documenting the ground we walk on, but my favourite has to be MSRY's Archive of Japanese Access Covers (sewer covers).
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You can view the entire archive of 2400+ photos on their Flickr site.
Some similar links:
Written In Stone is a gallery of sidewalks and the stamps and seals left by the people who laid them.
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Here's another for Oakland, California:
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And Denver, Colorado:
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