The P-38 shown in this photo was one of the fighters built in the late 1930s
and early 1940s that experienced compressibility effects. In steep dives,
these aircraft could reach speeds above Mach 0.75 (called transonic). At
transonic speeds, air in front of the wings became compressed and reached
supersonic speeds as it flowed over the wings, forming a shock wave. This
resulted in an increase in drag and a decrease in lift.

Another result was the movement of the wing's center of lift to the rear,
forcing the aircraft to rotate so that the nose moved downward and it went
into a steep dive. Pilots found that that their aircraft would not pull out
of this dive. When they attempted to pull out, they found the control stick,
as one pilot put it, "was cast in about two feet of concrete." In some
cases, the airplanes crashed or broke up in the denser air as they
approached the ground. In other cases, the pilots were able to pull out of
the dive. These accidents and near misses reinforced the popular belief in a
"sound barrier." The need for data at speeds near that of sound and the
inability of wind tunnels at the time to provide it would lead to the
construction and flight of the X-1 and D-558 research aircraft.