Workers in the sewers under the streets of Minneapolis heard a tell-tale alarm last August: sniffer devices detected a potentially dangerous explosive vapor. It was Aug. 8 — just weeks after gases ignited underground near the University of Minnesota on June 30, touching off a blast in a fraternity house, blowing manhole covers out of the street and prompting emergency evacuations and road closures. And it was less than a week after a second high-profile alarm about petroleum in the sewers, and subsequent evacuation in the same area. Those involved in the investigation described an exhaustive, multi-agency effort to pinpoint a cause as students flocked back to the U of M campus for the fall semester. They now believe they know where that volatile substance came from, and they’ve taken a wide variety of steps to prevent a recurrence.
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