There are two tunnels connecting the western region of Alameda to Oakland. These are the Posey Tube and the Webster Tube. They are both one-way, underwater tunnels. The Posey Tube goes from Alameda to Oakland, and the Webster Tube is from Oakland to Alameda.
Each tunnel consists of two vehicle lanes, in addition to an elevated pedestrian/bike path along the wall. It should be noted that the walls of the tunnels are very dirty from car exhaust, so it is best to avoid brushing against the walls if possible.