There are four significant underwater rock formations in the San Francisco Bay: Blossom Rock, Arch Rock, Harding Rock and Shag Rock. Located outside the main shipping channel, near Alcatraz and the Golden Gate Bridge, these rocks have been periodically dynamited since the 1860's to improve the navigability of the bay. All four are currently between 35 and 49 feet below the low-tide line with pending proposals to reduce them to 50+ feet below the low-tide line.