"I heard a loud popping sound ... about three or four minutes before ... then it blew open," Passenger Greg Hanson told CNN Affiliate KOVR.
"You can see the blue sky from your seat," he said in a phone interview from the plane.
Hanson described the hole as being about 3 or 4 feet long and about a foot wide.
The aircraft's crew confirmed to the Federal Aviation Administration that there is a hole in the fuselage.
Hanson said that he and the rest of the passengers were still on board Southwest Flight 812, after making an emergency landing at Yuma Marine Corps Air Station/International Airport at 7:07 p.m. ET.
The FAA said the plane experienced a rapid controlled descent from cruising altitude to about 11,000 feet after the cabin lost pressure. Investigators are en route to the base, the FAA said.
"We do not know the cause of the decompression," said Ian Gregor of the FAA.
Southwest said in a statement that the flight crew "discovered a hole in the top of the aircraft." (read more)