On July 2, 2007, Employee #1, a farmer, was pumping liquid cow manure from a covered holding pit. The pump clogged, and he descended the ladder into the pit to free the pump. Employee #1 was asphyxiated due to a lack of oxygen and the presence of methane from the manure.
Employee #2, a farm hand, entered the pit to rescue Employee #1 and was also asphyxiated.
Three family members of Employee #1 went into the pit to try to rescue the two employees and were also asphyxiated, resulting in five deaths.
The pit had to be ventilated before rescue personnel could recover the bodies. Employee #1 and #2, along with the three family members, were pronounced dead on the site by a local medical examiner.
Source: https://www.osha.gov/ords/imis/accidentsearch.accident_detail?id=200924876