WASHINGTON -- In passing references scattered through once-classified documents and cryptic public comments by former intelligence officials, it is referred to as "Midwest Depot," but the bland code name belies the role it has played in some of the C.I.A.'s most storied operations.From the facility, located somewhere in the United States, the C.I.A. has stockpiled and distributed untraceable weapons linked to preparations for the Bay of Pigs invasion and the arming of rebels and resistance fighters from Angola to Nicaragua to Afghanistan.