Ever wonder why there's a tiny hole at the bottom of airplane windows? Recently, Robbie Gonzalez of i09 went searching for answers to this question, putting any other hole-related theories to rest.

The first thing you need to know is that an airplane window is made up of three panes: the outer pane which functions as the “primary structural window;” the middle pane, where the hole is located; and the inner pane, the one closest to the passenger. Between the outer and middle panes is a small space called the “air gap.” Together, the two panes and the gap constitute the “two-pane air-gap design,” which is prepared to take on the full force of atmospheric pressure.

At cruising altitude—around 35,000 ft—the air pressure is 3.4 pounds per square inch. This is much too low for a human to function normally, so the air pressure in the cabin is maintained at around 11 pounds per square inch. According to Gonzalez, “The bigger the pressure differential between air outside the plane and air inside the plane, the bigger the strain placed on the plane’s various cabin structures, including its windows.”

In order to regulate the air pressure, the "breathing hole," as it’s officially called, acts “as a bleed valve, allowing pressure between the air in the passenger cabin and the air between the outer and middle panes to equilibrate.” It ensures that the full force of the air pressure only acts on the outer pane, leaving the middle pane available for emergencies.

To learn more about the tiny hole and its not so tiny responsibility, check out Gonzalez’s article on i09.