It's not yet known why a massive wooden roof collapsed overnight at a waterpark in Ottertail, Minn., authorities said. No injuries were reported, but some 25 to 30 people staying at the Thumper Pond Resort evacuated the facility, which was then shut down for the day. 

Someone reported hearing something that sounded like an explosion around midnight when the roof collapsed, but investigators with the Ottertail County Sheriff's Office said they found no evidence of an explosion, and attributed the sound to a support beam snapping.

The roof is less than eight years old, according to Thumper Pond general manager Brad Stevens. 

"It's very fortunate that no one was hurt," said Lt. Barry Fitzgibbons of the Ottertail County Sheriff's Office. He said the cause was not criminal, but engineers were examining the fallen roof Wednesday to determine exactly what happened. 

The resort, located 34 miles from Fergus Falls, advertised an 18-hole golf course, hotel and waterpark with a three-story tower and two slides.   

Photo: Thumper Pond Waterpark in 2009. Photo credit Glen Stubbe, Star Tribune.