Ships became increasingly larger and specialized in the second half of the 20th century. This led to the development of general cargo ships, tankers, grain carriers, barges, mineral carriers, bulk carriers, methane carriers, and container ships. Tankers, which were built to carry the enormous petroleum traffic of the post-World War II era, are extremely simple in design. Machinery is concentrated at the stern and virtually all the remaining space is devoted to compartments for liquid cargo.
The growth in ship size is usually a stepwise process. In 1959, the 100,000 dwt barrier was breached with the delivery of the Universe Apollo. By 1975 a VLCC (Very Large Crude Carrier) was more than 300 meters long, twice the length of a 1942 T2 tanker of 150 meters. The T2 tanker could carry about 141,200 barrels (5,930,000 gallons) and had a deadweight tonnage of about 16,000 tons. VLCCs introduced in the 1970s had a deadweight tonnage between 150,000 and 300,000 tons and could carry between 800,000 and 2 million barrels. Physical limits have however been reached in 1980 with the ULCC Seawise Giant, having a dwt of 564,739 and a draft of 24.6 meters. No larger tanker ship has since then been built.