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I have no idea if someone already posted about this, but I just saw this images and I loved them, went to look about it and had to share.
"A cenote is a natural phenomenon, a sinkhole in the Earths surface", the Yucatan peninsula has about 7000 and some are wonderful. You can go there and visit or swim. You can have an underground lake, but open on top.
I think it`s really beautifull and really want to visit one day!!
*Picture and quote are not mine but from link bellow* I hope I have my own pictures there to post one day! :)