Technically, there are two basement corridors in the main building. Although the two are parallel with each other they are at different heights. To avoid confusion the upper one is referred to as the locker corridor, creatively named as (unsurprisingly) it houses a number of lockers.
The "lower" corridor consists of some technology rooms (Rooms 37, 38, 39 and 41), both staffrooms, the Server room, the ICT Support office, Mr Grehan's office, Dr Jackson's office and Mr Seddon's office.
The Switch Room can also be found here - this is where the Welfare Staff actually live too, it is full of cleaning equipment and some Dr Frankenstein type, rather scary looking electrical switches.