Southwest Airlines Co. inspected about 200 planes overnight after a football-sized hole opened up in the passenger cabin of a jet in flight, forcing an emergency landing in West Virginia.

Travelers on the Boeing 737 aircraft could see through the 1-foot-by-1-foot hole that appeared during the flight Monday. The cabin lost pressure, but no one was injured on the Nashville-to-Baltimore flight with 126 passengers and five crew members on board.

Passenger Brian Cunningham told NBC's "Today" show Tuesday that he had dozed off in his seat in mid-cabin when he was awakened by "the loudest roar I'd ever heard."

Click to see a photo of the hole snapped by a passenger

He said the hole was above his seat. People stayed calm and put on the oxygen masks that dropped from the ceiling.

"After we landed in Charleston, the pilot came out and looked up through the hole, and everybody applauded, shook his hand, a couple of people gave him hugs," Cunningham said.

It's not clear what caused the damage.

The incident occurred just four months after Southwest agreed to pay $7.5 million to settle charges that it operated planes that had missed required safety inspections for cracks in the fuselage.

Southwest spokeswoman Marilee McInnis said the airline inspected 200 Boeing 737-300-series jets overnight at hangars around the country and discovered no other similar problems.

"It was a walk-around visual inspection just to check for structural integrity," McInnis said.

All those planes were put into routine service Tuesday morning, while the airliner that landed in West Virginia remained there. Representatives from the National Transportation Safety Board and the aircraft manufacturer Boeing were helping to determine cause of the hole, McInnis said.

Southwest was operating a normal schedule of flights — about 3,300 per day — with no cancelations or delays through midmorning, McInnis said.

The hobbled airliner was placed in service during the 1990s and went through "routine maintenance" this month, McInnis said.

The 137-seat 737-300 makes up about one-third of the carrier's fleet of about 540 jets, all various models of the Boeing 737.

In March, Southwest agreed to pay a $7.5 million civil penalty imposed by the Federal Aviation Administration for operating nearly 60,000 flights in 2007 on planes that had not undergone required inspections for cracks in the fuselage. About 1,450 flights took place after the FAA had notified Southwest of the missed inspections.

The settlement with the FAA also required Southwest to increase oversight at companies to which Southwest outsources maintenance work, and to give FAA inspectors more access to information used in tracking maintenance activities.

Dallas-based Southwest carries more than 100 million U.S. passengers a year, more than any other airline.

Click for more on this story from the Charleston Daily Mail