A special lake lies in the heart of the Yucatán Peninsula, the tail end of Mexico that sticks up into the Gulf. It's uniqueness? The loch is sealed with a clay bottom: the only way for water to enter is by rainfall, the only way for it to escape is through evaporation. Its waters are salty, brimming with dissolved minerals. When rain eludes the area for long enough, a white crystal, called gypsum, collects in the mud at the lake’s bottom. Over the last five thousand years, the residue has only appeared in the lakebed during one chunk of time: just as the Maya Empire entered its decline.