Worn-down manhole, probably a repository for abrasive gravel, as it is below the level of the road.
"A manhole (alternatively utility hole, cable chamber, maintenance hole, inspection chamber, access chamber, sewer hole, smellhole, flabhole, ding-dong or confined space) is the top opening to an underground utility vault used to house an access point for making connections, inspection, valve adjustments or performing maintenance on underground and buried public utility and other services including sewers, telephone, electricity, storm drains, district heating and gas."
"Manhole closings are protected by a manhole cover, a flat plug designed to prevent accidental or unauthorized access to the manhole. Those plugs are traditionally made of metal, but may be constructed from precast concrete, glass reinforced plastic or other composite material (especially in Europe, or where cover theft is of concern)."
"Manholes are usually outfitted with metal, polypropylene, or fiberglass steps installed in the inner side of the wall to allow easy descent into the utility space. Because of legislation restricting acceptable manual handling weights, Europe has seen a move toward lighter weight composite manhole cover materials, which also have the benefits of greater slip resistance and electrical insulating properties.
The access openings are usually circular in shape to prevent accidental fall of the cover into the hole."(Wikipedia)
Images of manhole covers in other countries: https://en.wikipedia.org/wiki/Manhole_cover