The cities of the ancient Maya in Mesoamerica never fail to impress. But beneath the soil surface, an unexpected danger lurks there: mercury pollution. In a review article in Frontiers in Environmental Science, researchers conclude that this pollution isn't modern: it's due to the frequent use of mercury and mercury-containing products by the Maya of the Classic Period, between 250 and 1100 CE. This pollution is in places so heavy that even today, it pose a potential health hazard for unwary archeologists.