Gowanus Canal: St. Mary's Playground
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. (VanishingNewYork)

Ninth Street Subway Viaduct curving over Smith St

Culver Viaduct curving over Smith St

St. Mary's Park

St. Mary's Park

St. Mary's Park

St. Mary's Park

Alley off Nelson St

St. Mary's Roman Catholic Church - Court Street

St. Mary's Roman Catholic Church

496 1/2 Court St

2nd Ave at 5th St

Pumping station

Warehouses on the northwest bank of the canal just south of 3rd St

Looking west down 5th Street from 2nd Ave toward the canal

Worldwide Tours / Great Escapes bus garage - 33 2nd Ave

Omega Express bus garage 35 2nd Ave

Looking west on unnamed street between the 4th and 7th Street basins

Looking south on 2nd Ave from 7th St

Architectural Grille / Rock Salt Depot - 53 2nd Ave

Looking east down 8th Street from 2nd Ave

Eagle Clothes Sign - 6th Street btw 2nd/3rd Aves

Looking up 3rd Ave from 6th St

Round warehouse window - 6th Street @ 3rd Ave

Looking west down 6th St

Dykes Lumber

6th Street Iron and Metal

Metal recycling - 6th St

Metal recycling - 6th St

US Recycling - 141 6th St

US Recycling - paper

Looking east down 6th Street from 2nd Ave