OAKLAND, Calif. (KGO) -- The homeless are building their own homes underneath the BART tracks in Oakland.Like along San Leandro Street near 81st Avenue, where at least a dozen wooden structures are tucked right under the tracks with trains whizzing by above at regular intervals.Structures that have clearly taken a while to build and don't appear to be going anywhere, anytime soon."It is a concern and the structures are an even bigger concern," said Joe DeVries, Oakland's Assistant to the City Manager."They're not a tent, they're not mobile. And as people build them up there's a whole new chapter around what we're concerned about around danger associated with encampments."Homeless encampments underneath tracks and freeways have sparked several fires in recent months.Last year, a man died inside his wooden structure after a fire near the BART tracks, underneath the 980 freeway.But the buildings just keep going up.Like near the BART tracks on East 12th Street, there is multi-story structure, made mostly of plywood, complete with windows and a front door lock.The residents didn't want to talk with ABC7 News, but a neighbor told us off-camera that he's reported the illegal structure to BART police several times in recent months, but it just keeps getting bigger.BART Spokesperson Anna Duckworth told us the district does own the property along San Leandro street, but not every location where the structures have been built."We're working with the city of Oakland right now," said Duckworth. "I mean the real issue is we need a place for people to go, so that's what's being worked out now."