OOLOGAH, Oklahoma - The loud, mysterious sound that caught people off guard in the Oologah area last week was made by a routine operation conducted at the PSO plant just outside of town, a spokesman confirms.

The sound is fairly common, happening every few months usually late at night or early in the morning.

It usually consists of a series of jet-engine like roars, each lasting for four or five seconds, for a total of about 30 seconds. Some people who live in the area posted on social media that the sound makes them think of a dragon.

Long-time residents of Oologah got used to the sound years ago and barely notice it anymore. But on Friday, May 18, 2018 it surprised people who are new to the rapidly-growing town. The roars started at about 10:15 that night and lasted for about five minutes.

Dozens of people took to a Facebook page used by Oologah residents to ask what the sound was. The sound is clearly audible in a video shot in Oologah, about a mile from the plant.

A spokesman for PSO said the sound was caused by a relief valve on one of the power generating units. The plant has two natural gas-fired units and one coal-fired unit (a second coal-fired unit was deactivated a few years ago). 

The gas-fired units are Unit 1 & 2 while the remaining coal-fired unit is Unit 3. The plant manager says the roars last Friday night happened when Unit 2 was taken off line and Unit 3 was starting back up after being shut down for maintenance. 

All the units heat water into steam which then spins generators to make electricity. When a unit is shut down, the leftover pressure in the system has to be released. That release causes the roars. 

PSO says the process lasted longer than usual last week because this particular shutdown process involved additional turbine testing.