Among many other things, whenever summer rolls around Philly likes to boast that it has the most public pools per capita than any other U.S. city. Indeed, it has 70 outdoor pools scattered around the city.

But that number doesn’t include the forgotten natatoria (Latin for buildings with indoor pools) that were once popular swimming spots in Philly during the 19th and 20th centuries.

Perhaps the most highly-regarded natatorium was the Philadelphia Natatorium and Physical Institute, located in the heart of Center City at 219 South Broad Street. The four-story, brick and concrete building opened in 1858 as a recreation center for the elite, complete with culinary services on the fourth floor, a banquet hall on the third, a dance hall on the second , and a gym and natatorium on the first floor.

The ornamental two-story natatorium was described by a “teacher of physical culture and swimming” in 1892 as follows:

It was 30 feet wide, by 100 feet long, from 3 to 9 feet deep, three feet on one end, sloping to nine feet at the other; the bottom and sides are of stone work and cement. There are seven pairs of stairs leading into it at different parts of the pool, railways around it, spring board to jump from, a sliding board from the gallery or floor of the second story leading down to the water.

Over its history the natatorium became well-regarded and highly-touted as “one of Philadelphia’s most important institutions,” according to one advertisement. It stated:

For many years the swimming school on Broad Street, below Walnut, has been patronized by the sons and daughters of Philadelphia’s best families. Thousands who to-day[sic] enjoy the luxury of swimming, owe the mastery of the art to the lessons they received at the Natatorium.

In 1916, the natatorium closed its doors and was converted into a hotel. In later years, it served as a parking garage. Today? A 15-story Cambria Hotel is rising in its place.

But although the Philadelphia Natatorium is long gone, another indoor swimming pool from later years continues to hide away underground along the Schuylkill River.

Known as Kelly Natatorium, this indoor swimming pool remains vacant and tagged with graffiti beneath Fairmount Water Works. It opened in 1961 as an Olympic training facility and later a public pool thanks to funding from Grace Kelly’s family (hence the name). But years prior it was an aquarium—at one point the fourth largest in the U.S.

The Kelly Natatorium only remained opened for 11 years, before closing in 1972 after it was damaged from a hurricane. It’s vacant to this day, although the rest of Fairmount Water Works has since re-opened and undergone renovations and restorations.