The Downtown Seattle Transit Tunnel (DSTT), also referred to as the Metro Bus Tunnel, is a pair of public transit tunnels in Seattle, Washington, United States. The double-track tunnel and its four stations serve Link light rail trains on Line 1 as it travels through Downtown Seattle. It runs west under Pine Street from 9th Avenue to 3rd Avenue, and south under 3rd Avenue to South Jackson Street. Line 1 trains continue north from the tunnel to the University of Washington station and south through the Rainier Valley to Seattle–Tacoma International Airport as part of Sound Transit's light rail network.