By Sid Perkins
About 2,000 years ago, the Roman geographer Strabo wrote about the residents of Latakia, Syria, who rowed their boats 4 kilometers out into the salty Mediterranean, dove a few meters to the ocean floor, and collected fresh drinking water in goatskin containers for their city. No miracle, this—marine boaters could do the same today at a spot about 10 km east of Jacksonville, Fla. In fact, similar freshwater springs erupt on the seafloor near many shores. These flows of water originate on land and end up in the sea, just as rivers do—only they take a subterranean route to their destinations.
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![](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2005/10/5344.jpg?resize=300%2C109&ssl=1)
Such underground rivers form only under certain geologic conditions. At some sites, the water flows from onshore aquifers to the sea through porous rocks and then seeps up through the seafloor. At many more locales, groundwater drains at low tide through sand or other porous shoreline sediments into the ocean.