In the 1910's the Catskill Aqueduct engineers designed the system to have periodic drains so segments could be cleared and inspected. Now a hundred years later, and with the new third tunnel complete, maybe that will begin to happen!
The grid design was a standard in the late 1900's and the designer added a small square in each coffer to keep things peppy.
This is a drain of a major distribution main emerging from the end of the aqueduct near Ft. Greene Park.
This example is on State St. between Third and Flatbush Avenues in Brooklyn. There are many similar covers throughout the city.