abstract:The Mach Loop (also known as the Machynlleth Loop) refers to a series of valleys in the United Kingdom in west-central Wales, notable for their use as low-level training areas for fast jet aircraft. The system of valleys lies 8 miles east of Barmouth and is nestled between the towns of Dolgellau to the north and Machynlleth to the south, the latter of which it takes its name from.