Volkswagen's "secret" speed test track.
Whether that be Top Speed or Acceleration.
Essentially, it's a giant oval with two wide bends and a giant straight and is one of the few places on Earth to even get to the real top speed of some cars.
It was known for having got the Top Speed records of the Bugatti Veyron EB 16.4 and Bugatti Veyron EB 16.4 Super Sport as well as the McLaren F1.
It has a strict no fly zone policy to keep Volkswagen or Volkswagen permitted manufacturers' prototypes a secret.
Here's a photo: