The commercial 737-800 jet was about 9 miles away from Will Rogers World Airport early Wednesday morning when it suddenly plummeted and was observed flying dangerously low over the city of Yukon ... reaching its lowest altitude over the city's high school.

An air traffic controller alerted the Southwest pilots of the low altitude after an automated warning sounded off just after midnight. In an audio transmission, you can hear them asking, "Southwest 4069, low altitude alert. You good out there?"

One of the pilots responds, "Yeah, we're going around, 4069" ... before the air traffic controller instructs them to maintain 3000 feet.

It was later confirmed the plane missed the approach when it first tried to land -- so it's unclear if this is a Boeing issue, or human error. After its too-close-for-comfort approach, it eventually regained altitude and reportedly circled around the airport before landing safely.

Southwest is launching a thorough investigation alongside federal officials, pledging to unravel any irregularities with the aircraft's approach to the airport -- all while reaffirming their unwavering dedication to the safety of both passengers and crew.

Meanwhile, local residents were left trembling with fear as they witnessed the plane flying uncomfortably close ... with one resident recounting to a local news outlet how the intensity of the plane's sound jolted him right out of bed.