Commercial exploitation began in 1872, and by the beginning of the 20th century the Baku oil fields were the largest in the world. Towards the end of the 20th century, much of the onshore petroleum had been exhausted, and drilling had extended into the sea offshore. By the end of the 19th century skilled workers and specialists flocked to Baku. By 1900 the city had more than 3,000 oil wells, of which 2,000 were producing oil at industrial levels. Baku ranked as one of the largest centres for the production of oil industry equipment before World War II. The World War II Battle of Stalingrad was fought to determine who would have control of Baku oil fields. Fifty years before the battle, Baku produced half of the world's oil supply.