Hazmat Disposal Site L5 is the name given to a system of caves storing radioactive waste that connect L'Enfant Plaza to The Mall. Entrance may be gained through the L'Enfant Plaza Metro tunnels. A hole in the tracks leads you to the cave network below the subway tunnels. Another entrance is located at the Southwest corner of The Mall, near the Lincoln Memorial.
The disposal site is home to one or two Super Mutants (Near the L'Enfant Plaza exit) and a number of Ghouls--including a small number of Glowing Ones and possibly a Reaver at higher levels with the Broken Steeladd-on. There are also a number of bodies. Other than what you may glean from them, the Site possesses no treasure of note, serving a merely connective purpose.
Upon entering and subsequently leaving the Hazmat Disposal Site, a character might not fully (depending if they were in Sneak or regular mode) revert to regular mode if in sneak, or not go into sneak mode if in regular mode. This can sometimes be fixed by reloading the autosave made after leaving the Metro.