Sewer snake got him. They live in the sewers, and burrow up through the ground. Their open mouths look just like open manholes. They wait motionless for prey to fall in, sometimes for hours. Eventually the snake goes away, and when it's safe, a city worker comes along and coveres the hole over with a steel plate. That's why there are so many manhole covers in the cities -- they aren't all manhole covers, some of them are covers for holes made by sewer snakes.