In Philip Reeve's Mortal Engines and Predator's Gold, Traction Cities are vast metropolises built on tiers that move on gigantic caterpillar tracks. These cities hunt smaller cities which in turn hunt towns which in turn also hunt static settlements in a practice known as Munincipal Darwinism. There are also aquatic equivalents called Raft-Cities which travel in the oceans hunting smaller raft suburbs and small islands. A few Raft Cities are Puerto Angeles, Grimsby and Marsailles. There is also a Traction city that travels in the skies called Airhaven which travels around the world trading. Traction cities included London, Paris, Panzerstadt-Bayreuth, Tumbridge Wheels, Wolverinehampton, Arkangel and Anchorage. These municipalities were formed after the Sixty Minute War which resulted in the deaths of millions, a barren earth and the loss of most technological knowledge. After the war, many people and their municipalities were mobilised into vast tiers that move on caterpillar tracks. Their worst enemy is the Anti-Traction League which opposes Municipal Darwinism and Traction cities.