Likely
methane gas at this TX campus. This is why we'd always run a sniffer down into the sump area of the pump stations located at freeway underpasses before we'd go down the ladder to perform maintenance. Also, we went in pairs, one guy stayed topside and the guy that went down below had a rope tied around his waist and was in constant radio contact. If 2 men were needed below, 3 men went to the station.....IOW, a man was always topside. Not only is methane flammable it displaces oxygen in confined spaces.