If you hear a bang in the night in Oklahoma, it's probably not a monster, but could something worse -- an earthquake.
Across the south central state, 20 earthquakes were reported to The United States Geological Survey on Saturday alone. One of those quakes in the Edmond area had a magnitude of 3.5.
But residents are puzzled as to why the quakes are occurring so frequently and making such alarmingly loud noises.
"Felt like bombs going off. It's just a huge loud noise and then it's like a reverb from that boom that just shakes the entire house," Logan County resident Nancy York told ABC News affiliate KOCO-TV in Oklahoma City.
"If I'm experiencing eight of these in one day, then when does it erupt and become absolutely horrible that takes my house down?" York asked.
Similar booms have been heard across other states including Indiana, South Carolina, Ohio and Rhode Island in the last month. All of these regions are active areas on the USGS seismic hazard map.
Seismologist Austin Holland with the Oklahoma Geological Survey said that the unexplained noises are a result of sound waves emitting deep from within the earth before erupting. Residents might not know it's an earthquake because the shaking may be too slight to detect.
"When you're on top of a small earthquake it generates a boom. It's kind of similar to an explosion," Holland said. "I know these booms have been reported in other places and they couldn't figure out what was going on."
Oklahoma has seen a steep rise in the frequency of earthquakes in the area, according to a joint statement by both USGS and OGS. Residents have experienced more than 200 measuring at least a magnitude 3.0 since the beginning of 2009.
Holland said the USGS and OGS are conducting joint research on the increase in the frequency of earthquakes in the area.
It is not clear if the quakes are being triggered by human activity or are occurring naturally, Holland said, although he said suggested changes to lake levels may be involved.
The scientists are looking at hydraulic fracturing among all other possible factors, he said, but it may be a while before they come up with a definitive answer.
"We have no way to predict the future. Earthquakes aren't predictable," Holland said. "Certainly the more earthquakes we have, the more likely we are to have a larger one."