St. Mary's Playground is a parks facility under the 9th Street subway viaduct.
It is named for the nearby St. Mary's Star of the Sea Roman Catholic Church,
which was founded in 1851 and given a name common to sea-faring communities.
The viaduct for the IND Crosstown line opened in 1933. The land was
acquired by the NY Dept. of Transportation in 1934 by the WPA with
the intention of using it for the proposed Gowanus Expressway. The
expressway was routed to the south (opening in 1941) and in 1955 the
land was transferred to the Parks Department. The first playgrounds
opened in the late 1960s.
Meanwhile on the far more industrial east side of the canal,
just north of the corner of 5th Street sits a small subterranean pumping station
that seems to be unrelated to the Butler/Douglass Street pumping station.
Although I haven't found any reference to the 5th Street pumping station in the Army
Corps of Engineers documentation or online, I could definitely hear water gurgling
through grates in the sidewalk, and I presume that the water flushes the
4th and/or 6th Street basins.
Eagle Clothes opened a plant on 6th Street between 3rd/4th Avenues in 1951.
They filed for bankruptcy in 1989, although their sign has remained,
perhaps because its sturdy construction will require considerable effort to demolish.