On August 24, 2008, at 1830 central daylight time, a Piper PA-28-161, N523PU, collided with an open manhole while taxiing after landing at the New Ulm Municipal Airport (ULM), New Ulm, Minnesota. The private pilot and one passenger on board were not injured. The airplane sustained substantial damage to the wings. The personal flight was operating under Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed. The flight originated from the Flying Cloud Airport (FCM), Eden Prairie, Minnesota, at 1740. The pilot reported that after landing he was taxiing into a tie down spot on the ramp when the left main gear dropped into an open manhole in the grass near the edge of the ramp. He stated that because the tie down spaces are so close to the edge of the ramp, he had to taxi into the grass in order to make the turn into the tie down space. He did not see the hole prior to contacting it, nor was the open hole marked in any way. The airport manager stated the manhole cover was a concrete, 150 pound, 4 foot by 4 foot by 3 inch high lid that covered the hole containing the electrical junction box for the fuel pumps. He stated that the fuel company and a local electric company had worked in the manhole several weeks prior to the accident. He stated he assumed they replaced the cover over the hole when they were finished with the work, but the cover was still on the grass next to the hole. The cover and the hole were not visible from the ramp due to the grass. The airport manager stated that one of the airport line personnel had seen the cover off of the hole, but did not report it to him or anyone else. The airport manager stated that following the accident, he contacted the company that had been working in the hole, and was told that the cover was too heavy for one person to lift which is why it was not placed back over the hole.