Underground river beneath a City, Phiadelphia

In the early 1800s, Mill Creek was a working stream. It rose in Montgomery County, entered what became West Philadelphia near today's City Line Avenue and 63rd Street, and ran about five miles southeast to the Schuylkill River, powering sawmills and gristmills along the way. It was fast, and it flooded. After the Civil War, as the city laid its street grid over West Philadelphia, planners decided to put the creek underground. Beginning in 1869, crews enclosed it in a brick sewer about 20 feet across and buried it under landfill up to street level. In places the fill sits as much as 35 feet above the old creek bed. Landscape historian Anne Whiston Spirn later gave the result a name: a "buried floodplain." Rows of houses were built directly on top of it, and a 1927 atlas shows the sewer's curving line running under the homes. The creek didn't stop being a creek. As West Philadelphia and the suburbs upstream filled in with pavement and buildings, more and more stormwater and sewage poured into a pipe too small to carry it. The sewer carries the waste and stormwater from half of West Philadelphia and from suburbs upstream. Under that pressure it overflowed, and sometimes it gave way. In the early 1940s, 47 homes above the sewer were demolished because they were overrun with rats and sewer gas. In 1945, a block of row homes built over the sewer on the site of a former mill was destroyed when the pipe collapsed. In 1952, a cave-in swallowed a car and a pickup truck, and after more rain the street kept sinking, taking the front porches of dozens of homes. The worst came on July 17, 1961. The sewer caved in beneath Funston Street near 50th Street. Four houses were destroyed and three people were killed, including a nine-year-old boy. More than 100 homes along the sewer line were then condemned and demolished, leaving hundreds of people homeless. Many residents had bought their homes without knowing the neighborhood was named for a creek running beneath it. Today, an interceptor tunnel carries Mill Creek's flow to the city's Southwest treatment plant, but heavy rain still turns the culvert into a raging underground river, and flooding happens when water backs up out of the street drains. Much of the land where houses once stood has become woodland, meadow and community gardens, and one garden on Aspen Street has been growing on a former residential lot since 1975. Since 1987, Spirn's West Philadelphia Landscape Project has worked with residents, students and the Philadelphia Water Department to map the buried floodplain and plan green spaces that hold stormwater instead of fighting it. The creek has been out of sight for more than 150 years, and it's still running.