Georges Head was later chosen as the best place to spy and fire mines which were laid underwater. In 1888 Georges Head battery was upgraded for the latest in harbour defences in the form of submarine bombs. Minefields were laid across the main shipping channels of Port Jackson from 1876 to 1922 and a base was built at Chowder Bay for the submarine miners. From Georges Head, miners watched for ships entering the harbour. Their job was to explode the mine closest to an approaching enemy ship. Each underwater mine was attached to an electric cable that ran up the cliff to the firing post.