EUGENE, Ore. (AP) -- Crews at the University of Oregon are scrambling to return power to a number of buildings after a series of explosions rocked an underground tunnel.

Nobody was hurt in the Monday morning blasts, but the student health center and a number of residence halls were still dark hours later.

Campus police spokesman Kelly McIver says the cause is still unknown. McIver says there were about five explosions around 10 a.m., followed by a series of blasts for about half an hour.

Videos showed puffs of smoke coming from manhole covers, and the scent of burning rubber lingered in the air.

After power was shut down, fire crews went into the underground tunnel to confirm that no fires were burning.

Students say they're using flashlights to get around dark dormitories.