Several days ago I started reading The Old Ways: A Journey on Foot, in which the author "sets off from his Cambridge home to follow the ancient tracks, holloways, drove roads and sea paths that form part of a vast network of routes that crisscross the British landscape..."
I added "holloway" to my list of words to look up. Then by coincidence I encountered a recent post by Miss Cellania on The Sunken Lanes of Europe, citing Amusing Planet, which has a gallery of photos (whence the embed).
Details at Wikipedia:
A sunken lane (also hollow way or holloway) is a road or track that is significantly lower than the land on either side, not formed by the (recent) engineering of a road cutting but possibly of much greater age.Fascinating, both visually and conceptually.
Various mechanisms have been proposed for how holloways may have been formed, including erosion by water or traffic; the digging of embankments to assist with the herding of livestock; and the digging of double banks to mark the boundaries of estates. These mechanisms are all possible and could apply in different cases.