https://en.wikipedia.org/wiki/Mach_Loop
The Mach Loop (also known as the Machynlleth Loop) is a series of valleys in the United Kingdom in west-central Wales, notable for their use as low-level training areas for fast aircraft. The system of valleys lies 13 km (8 mi) east of Barmouth between the towns of Dolgellau to the north and Machynlleth to the south, from the latter of which it takes its name. The training area is part of the United Kingdom Low Flying System and lies within Low Flying Area 7 (LFA7), which covers most of Wales.[1]
vue d’en haut par un spotter au sol