Hydrogen Sulphide accident

 At 11:00 a.m. on July 7, 2017, Employee #1was attempting to dislodge a 24 inch rubber plug from a 2foot diameter sewer pipe located inside a 24foot deep wet well. The workers were outside the well pulling on a 1/4-inch nylon rope that was attached to the 24-inch diameter plug. The plug was lodged inside a T-shaped PVC fitting from the force of the waste water emptying into the well. Without conducting any atmospheric testing of the work space, Employee #1 climbed down the ladder with a crowbar to dislodge the deflated 24inch diameter rubber plug, which was about 8 feet below the top of the well. He had difficulty releasing the plug with the crowbar and started to make his way up the ladder. He lost consciousness when he was about 2 feet from the top of the well and fell into the 24 foot deep well. Employee #2 descended down the ladder to provide emergency rescue, but lost consciousness and went underwater. The waste water level was about 3 feet deep at this point. Employee #3 climbed down the ladder to provide emergency rescue, but consciousness as well. All three workers were asphyxiated by hydrogen sulfide (H2S) gas. 

Source: osha.gov