No offense, but pits are quite common. Typically there is a 6 ft deep pit under the entire building. The floor of the building is slats made of concrete (sometimes another material, but ususally concrete) which allows the manure to fall into the pit.
If the pits aren't ventilated properly occasionally methane gas builds up. That's probably what these guys ran into when they went in to unclog the pipe.
My sons were trained at an early age to NEVER go into the pit without first checking for potential problems.
Several years ago there were 3 generations of a farm family who died in a pit. A little boy fell in, his dad went in after him,, they got into trouble, so the Grandpa went in after them. They were all lost.
Farming is the 2ed most dangerous profession, coal mining is the first. (I think I've got that right. Open to a correction if I'm wrong)
Not that farming is not dangerous. My cousin lost both his arms in a farming accident when he was 16.