LaDainian Tomlinson -- SELLING INSANE MANSION ... Secret Room, Rope Bridge, Movie Theater

If you've got a spare $3.5 million laying around ... YOU GOTTA BUY LADAINIAN TOMLINSON'S MANSION ... it's for sale -- and it's unbelievable.

The 13-acre property sits on two lots in an ultra-private community just outside of San Diego ... complete with 10,000 square feet of living space.

But here's where it gets crazy ... the 5 bedroom, 6 bathroom mansion includes ...

-- A resort style pool with a ROPE BRIDGE and waterfalls
-- private basketball court
-- private golf area with sand trap
-- wall aquarium
-- wine cellar
-- it sits up against the steep hill where LT used to train during his NFL career

... oh, and also has a SECRET ROOM that's only accessible through a door behind a bookcase.

You gotta check out the video ... the bookcase opens up and leads to a private man-cave movie theater complete with stadium style seating and a '50s malt shop themed bar area.

LT bought the home back in 2004 for $3.5 mil ... and it's listed for $3.495.