(CNN) -- Southwest Airlines announced Saturday that it is grounding 79 planes for inspection after a 3- to 4-foot hole tore open in the fuselage of one of its planes, bringing a view of the blue sky and a white-knuckled emergency descent.

The plane -- which had been traveling from Phoenix to Sacramento, California -- managed to make an emergency landing at a military base in Yuma, Arizona, on Friday. There authorities "discovered a hole in the top of the aircraft," the company said earlier.

Investigators are trying to determine what caused the "depressurization event" and have "decided to keep a subset of its Boeing 737 fleet out of the flying schedule to begin an aggressive inspection effort in cooperation with Boeing engineers," according to a company statement.

"The safety of our customers and employees is our primary concern, and we are grateful there were no serious injuries," said Mike Van de Ven, Southwest's executive vice president and chief operating officer.

The airline initially reported that 81 of its Boeing 737 aircraft would be grounded, but later decreased the number of planes to be inspected for what it called "aircraft skin fatigue." The inspections will take place at five locations over the course of the next several days, the airline said.

Southwest expected to cancel about 300 flights Saturday to accommodate the inspections. It advised customers to check the status of their particular flight or rebook their trip before heading to the airport. They may "experience sporadic delays of up to two hours on some flights."

National Transportation Safety Board member Robert Sumwalt said a preliminary investigation showed the aircraft suffered a "structural failure" and "depressurization" that prompted the emergency landing.

A subsequent review will investigate the aircraft's structure, metallurgical aspects of the plane, flight data recordings and its maintenance records, he said.

"We're in the beginning stages of the investigation," Sumwalt added.

Meanwhile, some of the 118 passengers who were aboard the crippled Boeing 737 said they had feared for their lives.

"We were in shock," passenger Debbie Downey told CNN Saturday. "We were in row 16 and my husband and I could see blue sky ... the wiring, the cabling. It actually was terrifying."

She said. "a lot of people were crying and holding hands" but had trouble hearing due to the noise of air rushing through the plane's opening. "It was very, very scary."

"I heard a loud popping sound about three or four minutes before it blew open on us," passenger Greg Hansen said.

"(Then) a big explosion happened. A big noise, and from there, you felt some of the air being sucked out. It happened right behind me, in the row behind me and it covers about 2 1/2 rows," he said from seat 11C.

Hansen, 41, a regional sales manager for a biotech company, was flying home to Sacramento, California, from a business trip. Some people panicked and screamed as the blue sky and sun began to shine through the cabin in midflight, he said.

"Most people were just white knuckles holding onto the arm rests. The pilots did a great job and were under control to get us to a manageable level," he said.

But just behind him, Hansen said he could see the jagged edge of the aircraft where the rivets used to be.

"You can see the insulation and wiring. The interior ceiling panel was bouncing up and down with the air," he said.

"It was surreal, when you're riding in a modern aircraft. You're used to being enclosed and not having the window rolled down," he said.

Hansen described the hole as being about 3 or 4 feet long and about a foot wide.

Passenger Brenda Reese told CNN affiliates KCRA and KOVR she began to fear for her life.

"I was texting my sister to make certain that she told my kids that I loved them," Reese said.

Southwest Flight 812, which had a five-member crew, then made an emergency landing at Yuma Marine Corps Air Station/International Airport at 5:07 p.m. (7:07 p.m. ET.)

The Federal Aviation Administration said the captain made a rapid, controlled descent from 36,000 feet to about 11,000 feet after the cabin lost pressure.

"We do not know the cause of the decompression," said Ian Gregor of the FAA.

Hansen said the incident took place about 35 minutes into the flight. He said it took about 45 seconds or a minute before the oxygen masks came down after the hole blew open.

"The crew was pretty calm about it. They walked around and checked on everyone," he said. "But it wasn't like the movies where papers get sucked out of the hole, but you could feel it and hear the noise."

Hansen said most of the passengers were complaining of a pain in their eardrums from a rapid descent.

Southwest said Saturday at least two minor injuries were reported, but neither the flight attendant nor the passenger were taken to a hospital.

Hansen said one male flight attendant appeared to fall and was bleeding from a facial injury.

Yuma International Airport spokeswoman Gen Grosse said passengers were tended to and given refreshments because the temperature on the tarmac was close to 100 degrees.

Southwest said it provided a full refund, an apology and two complimentary round-trip passes on the airline for future flights.

The second flight landed in Sacramento on Friday night.

Boeing spokeswoman Julie O'Donnell declined to comment on possible causes of the incident.

"Boeing is committed to ensuring safe flight and to supporting our customers," the company said in a statement. "We are working closely with Southwest and are providing technical assistance to the U.S. National Transportation Safety Board as it investigates the incident."

An NTSB investigative team arrived Saturday in Yuma and entered the plane.

A total of 288 Boeing 737-300s are currently operating in the U.S. fleet, and 931 operate worldwide, according to the FAA.

CNN's John Branch, Rich Phillips, Ted Rowlands, Deborah Doft and Greg Morrison contributed to this report