The folks who used to live where I am now sure left a mess. For some reason, they carved holes in an awful lot of the granite. Some of the holes have, thankfully, filled with dirt and are no longer a problem, but the majority of them can still collect a considerable amount of rainwater. When the weather warms up, they become breeding holes for mosquitoes.
These people must have made mosquitoes a regular part of their diet, considering the abundance of nurseries they created. They probably ate the larvae rather than the biting adults, kind of like really teensy prawns.
If they weren’t mosquito eaters, maybe the holes were used to wash socks, but they’re only big enough for one sock at a time, and from what I’ve heard the previous tenants never wore socks anyway. You could float tiny model canoes, testing them like we do model aircraft in a wind tunnel, before going to the expense of making a full-size canoe. Maybe they were mirrors, attracting birds who would attack their reflections and drown, ready to be plucked, cooked and eaten.
Or maybe they’re not human-caused at all; they could be the footprints of the extinct cone-footed Heavisaurus pogostickus. Yeah, that makes the most sense.