The River Styx?

WHOI researchers Paul Henderson (left) and Matt Charette prepare to enter a cenote, or natural sinkhole, near Puerto Morelos, Mexico. The limestone bedrock in this region is very porous, with caves in many locations where the rock has been dissolved. These caves are ideal windows underground that allow researchers access to groundwater without having to drill into the rock. Charette and his colleagues study the cycling of nutrients in groundwater and the flow of groundwater into the coastal ocean.(Photo courtesy of Meagan Gonneea, Woods Hole Oceanographic Institution)