Slightly acidic rainfall dissolves limestone as it passes through the ground, and as water travels underground toward springs it can create large cave passages through porous limestone. When limestone breaks down below the surface, the earth can cave in and form sinkholes. Cave divers throughout north Florida have found old tires, construction debris, discarded fuel canisters, and other items that property owners dumped in sinkholes. Sinkholes can also form when rains follow a drought and when large wells are being constructed and used, causing groundwater levels to drop. This can happen dramatically: in 1998, about 75 miles south of here in Pasco and Hernando counties, hundreds of large sinkholes formed in six hours during installation of an irrigation well.