When I worked on survey crew as a young engineering student, we did a lot of sewer "pck up". especially when there were backups into buildings. The methane buildup in clooged manholes and lines can be dangerous shit.

We had no oxygen masks or any safety equipment. Only common sense. We would always have one man whose only job was to watch anyone who entered a manhole. Always had a rope tied on the guy in the hole. One day, the sun was at an angle so the boss had to get on his knees and stick his head into the manhole to assess the situation... yup, passed out and fell in. Pretty messy. And, the rope policy was altered.