The captain on the flight had 19 years experience with Southwest, while his first officer had seven years of experience, and he received assistance as typical when declaring a emergency, according to the NTSB.
The damaged aircraft had been through more than 39,000 takeoffs and landings -- each of which puts stress on an aircraft's skin.
"When airplanes take off and land, aircraft are pressurized. The internal cabin is expanded and contracts just a little bit," said Steve Ganyard, a former military pilot. "You're exercising the skin of that aircraft."
Other fuselage cracks were found and repaired on this particular aircraft during heavy maintenance in March 2010, according to The Associated Press, which originally reported there were at least eight cracks in the fuselage.
Upon review of the records of the repair, ABC News discovered at least a dozen fuselage crack repairs. This sort of damage is not unexpected for a 15-year-old Boeing 737.
Airlines conduct major plane teardowns for these exact reasons -- in order to hunt for and fix damage such as fatigue cracks.
The cracks do, however, indicate that the plane was showing its age.
Southwest said it is working with the NTSB and FAA to determine the cause of the "depressurization event," and the airline canceled approximately 300 flights on Saturday and another 200 today.
"The safety of our Customers and Employees is our primary concern. We are working closely with Boeing to conduct these proactive inspections and support the investigation. We also are working aggressively to attempt to minimize the impact to our Customers' travel schedules today," Mike Van de Ven, Southwest's executive vice president and chief operating officer said.
For the first 20 minutes of the flight, all appeared normal as the plane climbed to 36,000 feet. Flight attendants had just taken drink orders when passengers reported hearing loud pops.
Soon, the roof opened up. Astonished passengers described a gaping hole, which investigators now say is five feet long and a foot wide, right next to the luggage compartment.
The plane suffered a rapid decompression, oxygen masks popped out and the plane went into a dive, according to passengers and officials.
Passenger Wade Allemand said he almost passed out.
"Your ears instantly start to hurt really bad. You feel like you're going to black out," he said.
The Southwest pilots radioed air traffic control, declared an emergency, and began a rapid descent -- quickly diving to a lower altitude so passengers would be able to breathe on their own.
The jet descended from 36,000 feet, cruising altitude, to 11,000 feet in four and a half minutes. It leveled off at 9,000 feet.
Some terrified passengers clearly thought it was the end. One woman whose husband was on the plane received a text from him -- "Plane going down. Love you."
However, the plane was able to land at Yuma Marine Corps Air Station/International Airport at 4:07 p.m. Friday, said Ian Gregor of the Federal Aviation Administration.
Passengers applauded the pilots upon landing, called loved ones and waited for a new Southwest jet to pick them up and take them on their way.