“Plastic box” looks like a utility cover.
The utility cover is one used commonly for water valves / distribution systems (one pipe is split into multiple pipes). The hole that is lined up with the utility cover could be a water valve pit.
The question is - why do you need a water distribution valve in that area? I see nothing nearby that requires water (buildings, etc). It could be for a sprinkler system- but I couldn’t imagine wasting water on an island like that.