SOUTH HADLEY, Mass. – An all-terrain vehicle carrying six people plummeted over a cliff in total darkness, sending them tumbling as far as 700 feet but killing no one in what a fire chief called a miracle.

The large ATV went over the cliff in a wooded area of Skinner State Park around 1:30 a.m. Sunday, first falling 50 to 75 feet down a sheer cliff face, then tumbling the rest of the way down a steep slope covered in jagged rocks, authorities said.

“We’re absolutely amazed that no one was killed,” South Hadley District Fire Chief David Keefe said Monday. “It’s a miracle.”

Colin Macomber, 28, of South Hadley and Chloe Reynolds, 27, of Northampton were in serious condition Monday at Baystate Medical Center in Springfield. Billie Lyman, 28, and Greg Wilson, 28, both of South Hadley, were in fair and good condition, respectively.

Keith Simpson, 28, of Boston, and Kurt Wolfard, 29, of New York City, were treated and released at Holyoke Medical Center for less serious injuries.

ATVs are prohibited in the park. The cause of the crash wasn’t immediately clear and is still being investigated. It’s also not clear who was operating the vehicle.

None of the crash victims appeared to have been wearing helmets, said Catherine Williams, spokeswoman for the Massachusetts Environmental Police.

More than 50 rescuers from South Hadley, police and fire departments from surrounding communities, as well as state agencies, worked in the dark for about four hours to rescue all the victims, using ropes to scale down the cliff face, then pull the victims up once they had been treated and stabilized.

“Our guys had to get down there, package the patients in total darkness, bandage them, immobilize them, get them on a backboard, get them in a scoop stretcher, then get them up the hill,” Keefe said.

“We just did it with brute force. There was no time to finesse this,” he said.

The victims had puncture wounds, broken bones, head injuries, lacerations and other injuries.

Authorities were not alerted until Simpson, who was not familiar with the area, made it out of the woods to a home to call 911.

Keefe said it took his men, who are familiar with the terrain, about 15 minutes to get through the woods to the site on ATVs.