From a story tonight on BBC Science and Nature
About Chicxlub crater on Yucatan Peninsula

http://news.bbc.co.uk/2/hi/science/nature/2837407.stm

The collapse of numerous limestone
caverns above the crater rim has
resulted in an arcing chain of
sinkholes - also visible in the shuttle
radar image.

"They are the result of extensive
erosion in the limestone. These
beautiful cenotes, as they are called,
are several metres across and were
in fact used by the Mayans to make their sacrifices.

cenote suh no' tay
n.
5AmSp < Maya tzonot6 a deep natural well carved out of friable limestone