An undersea tunnel is a tunnel which is partly or wholly constructed under the sea or an estuary. They are often used where building a bridge or operating a ferry link is unviable, or to provide competition or relief for existing bridges or ferry links. While short tunnels are often road tunnels which may admit motorized traffic, unmotorized traffic or both, concerns with ventilation lead to the longest tunnels (such as the Channel Tunnel or the Seikan Tunnel) being electrified rail tunnels.