This is very confusing. I've been around farming forever and have never heard of a manure pit. Possibly a dairy farm where they spray down the floors and the waste is washed into a holding tank? The journalist didn't exactly follow through on the details, again. From the incomplete story, I'm more inclined to let the farmer off of murder charges and go with employing illegal aliens.
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)
We had a local hog farmer who was killed by this type of incident. They had a very large operation and one of the co- workers went into a "pit" to work on some pumps. He went down, so the hog farm owner went down to try to save him and he went down. Another worker, a co- owner showed up and found the two in the pit, had the piece of mind to not go in, but rather called for help. Both guys in the pit were already dead, the third would have been if he went in. Methane gas is what killed them. This comes under, in the fire service, confined space rescue. As an aside, I worked for this guy in one of my first jobs out of high school. I lasted 2 hours and couldn't take it anymore. I didn't even wait to get paid.