Many moons ago, I was instructing a student in the art of low level navigation at 100 feet.
Whilst passing over a large reservoir at said height, we were underflown by a Hunter [of the Fleet Air Arm ] doing about 400 kts.
Made my heart flutter a little. I intended to file an airprox on return to base until I discovered it was being flown rather exuberantly by an old mate of mine!!!