 After her commissioning , she served with the II Scouting Group , part of the reconnaissance forces of the High Seas Fleet . She was assigned to patrols off the island of Heligoland at the outbreak of World War I in early August 1914 . At the Battle of Heligoland Bight on 28 August 1914 , the German patrol forces were attacked by superior British forces , including five battlecruisers and several light cruisers . Mainz was initially stationed in support of the forces on the patrol line . She attempted to reinforce the beleaguered German forces , and encountered a much stronger force of British cruisers and destroyers . They scored several damaging hits with gunfire and a torpedo that disabled Mainz and prompted her commander to abandon ship . The British rescued 348 men from the crew before the ship rolled over and sank . Eighty @-@ nine men were killed in the battle , including her commanding officer . 
