Cover missing from an access hole - very dangerous

An access point in the middle of the road is missing the cover (seems to be water but the cover is gone so I don't know what it said). This occurred sometime in the last 3 weeks, as I've just noticed it on return from holiday. It is very dangerous as someone could easily break a leg if they were walking in the dark or were distracted in the day time. It is not underneath a street light and it is near a pedestrian cut through to the next road, so a lot of people walk this route.