On June 24, just after 11 am, a strange pulsating wave galloped across Oklahoma, averaging around 900 miles per hour and pinging seismometers across the state as it went.
As National Geographic reports, scientists soon realized that this wasn’t the first time the odd wave would move across the Sooner State, nor would it be the last. Researchers tracked down the source of the pulsating waves, which originated at the McAlester Army Ammunition Plant in Pittsburgh County, in the eastern part of the state. The strange waves were the result of munitions explosions, a common occurrence at the plant.
But what wasn’t so easily explained was how these explosions could send strange waves across the entire state. Scientists now believe the phenomenon is actually a pair of waves—one traveling through the shallow subsurface and another in the air just above the ground. Joshua Carmichael of Los Alamos National Laboratory believes “nature is conspiring to make the signals travel in such a way that we can’t tell whether they’re acoustic or seismic.”