What is the building located at 1800 Mission St (corner of 14th St) in
San Francisco? (It looks like a fortress, takes up nearly half a city
block, and has no signage on it.)

fly4fun...

From the National Register of Historic Places, it's the:

San Francisco National Guard Armory and Arsenal 
Also known as Coastal Artillery Headquarters & Training Base
San Francisco County - 1800 Mission St., San Francisco
(22 acres, 1 building)
	
Historic Significance:  Architecture/Engineering

Architect, builder, or engineer:
Woollett & Woollett, McLeran & Peterson

Architectural Style:  No Style Listed
Area of Significance:  Architecture
Period of Significance:  1900-1924
Owner:  Federal
Historic Function:  Defense, Social
Historic Sub-function:  Arms Storage
Current Function:  Vacant/Not In Use
http://www.nationalregisterofhistoricplaces.com/CA/San+Francisco/vacant.html


An article on the SF Mission website tells more:

"Site of one of the more memorable dot.com dot.gone battles
 in 2000. The Armory has been empty for decades. Consisting
 of 52 classrooms on Mission Street, a large gym suitable
 for college games and two huge floors underground which are
 often filled with water. Bob Kruezberger, local Mission
 historian, is frustrated that no would be developers ever
 discuss the Armory's history as a surgeon's college pre-UCSF.
 This structure, a historic landmark, is one of the most
 challenging development projects in the Bay Area. The 
 redevelopment of the 1914 brick structure at 1800 Mission
 St. has frustrated many over the years.

More on the page:
http://www.sfmission.com/theatres/

An article on the San Francisco Bay Guardian's website
details more about the development problems:
http://www.sfbg.com/News/34/51/51ogarm.html


A more recent article details the latest owner's plans
to turn it into a housing complex:
http://sanfrancisco.bizjournals.com/sanfrancisco/stories/2004/04/12/story4.html


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"1800 Mission St" "san francisco"
://www.google.com/search?q=%221800+Mission+St%22+%22san+francisco%22