Some of the earliest efforts to alter the flow of water in Florida began in the 1880s. Mineral prospectors discovered reserves of “river pebble,” which formed after thousands of years of deposits along the bottoms of river banks formed due to runoff. Dredging the material out of the rivers was a difficult process that destroyed the delicate ecology of waterways and their surrounding swamps. Prospectors later discovered larger deposits of “land pebble” 20 to 25 feet below the surface. Miners removed layers of vegetation and topsoil, in the process creating deep ponds filled by rainwater with little biodiversity. Even worse, the process of removing phosphate from the soil created copious amounts of slightly radioactive, acidic water that operators pumped into large refuse pools. Today these refuse pools line the southern region of Hillsborough County and oftentimes breach when large amounts of rain break the dikes and flood estuarine areas that line Tampa Bay. After the discovery of extensive phosphate mines in the "Four Corners Region" of Hillsborough, Hardee, Polk and Manatee counties, the Port of Tampa grew from a small fishing and import dock into a shipping powerhouse. This changed the ecosystem of Tampa Bay as the port dredged shipping lanes, destroying mangrove habitats, sea grasses and the ecology of the bay by depositing fill to construct large refuse islands.