Police showed up at the tunnel in droves just before 10 p.m. after getting word about a suspicious-looking device in the area.
The tunnel was closed and buses and trains were delayed while a Seattle Police Department arson and bomb squad checked out the device.
Members of of the squad originally thought the device might be dangerous, but after some careful investigation they figured out what it really was.
They now say the device was likely a crudely-designed lightsaber, which is a weapon from the fictional universe of the "Star Wars" movies.
Police say the lightsaber was made using a flashlight, a plastic toy, duct tape, a battery and string.