They are a complex of tunnels divided into three sections as ground districts which are accessible by manholes. Here vermin reside, as well as the ghoul rebels. Sewers are the only connection between Necropolis districts.
This is the southern part of the sewers. Underground ghouls lost control of this section to mole rats.
Hall of the Dead sewers
The underground ghouls reside here.
The Necropolis sewers appear only in the original Fallout.