An Xcel Energy substation at 14th and Jackson streets in east Denver burst into flames yesterday, sending a long plume of black smoke into the air (via the Denver Daily News). The explosion left an estimated 37,000 people without power, including many at Rose Medical Center, which experienced evacuations.

Power was expected to be restored to the outage area—from Quebec to Humboldt streets between First and 24th avenues—sometime this morning, reports The Denver Post, which points out that the incident left traffic lights inoperative, stranded dozens in elevators, and lured gawkers into the streets.

The explosion came at 6:30 p.m., when people likely had their air conditioners cranked on what was the hottest day of the year so far. The high temperature yesterday was 94 degrees, about 15 degrees higher than average for this time of year.

Xcel investigators have not yet identified the cause of the explosion.