GNDN line in service bay on the Enterprise
GNDN pipe was used throughout many Starfleet starships in the 23rd Century.
Background information
This is an old joke by the set designers on the original series. One of them put the letters "GNDN" on some pipe used on the Enterprise set. When asked what the letters stood for, he replied, "goes nowhere, does nothing," which was of course true.
This became a well-known enough prank that when the DS9 crew re-built some of the old Enterprise sets for the DS9 tribute episode "Trials and Tribble-ations", they made sure to include a GNDN pipe. Check the labeling on the white pipe in the lower left corner of the photo.