Do you know what happens when you use a road for hundreds of years without paving it? Foot traffic, horses, and vehicles wear away the soil, sinking the road down into the ground. Europe has many of these holloways, although a casual traveler might not see them because they aren't the well-maintained highways that will get you from here to there in a hurry. Instead, they are attractions in themselves.
Appearing as trenches dredged through the earth or tunnels cleared through forests, these ancient pathways called holloways or sunken lanes are found all across the European countryside. They originally began at the ground level, but over the centuries, under the tread of a million feet and hooves encompassing thousands of journeys, the floor of these roads have worn away and eroded down to the bedrock, creating ditches that lay beneath the level of the surrounding landscape. With high banks on either side, many of these ancient thoroughfare then became temporary waterways during rains, which further deepened and widened the paths making them permanent features of the landscape. Some of these paths are twenty to thirty feet deep, and look more like gorges than roads.
You can see plenty of photographs of the different holloways of Europe at Amusing Planet. -via the Presurfer
(Image credit: Romain Bréget)