A violent tornado touched down in northern Illinois on Thursday, April 9, roughly 80 miles west of Chicago, leaving many injured and killing two.

Preliminary reports estimated that at least two tornadoes touched down in a six-county vicinity, cutting a 50-mile swath of destruction. By Sunday morning, that estimate had risen to eight tornados.

Property damage in the area was extensive, as plows had to remove debris to allow emergency crews to reach the survivors.

Illinois Governor Bruce Rauner declared Ogle and DeKalb counties disaster areas, allowing the use of state resources for recovery efforts.

Tornadoes are one of the most destructive natural forces on earth, and Illinois is no stranger to their power.

The state has experienced two of the worst tornadoes in United States history: the Mattoon tornado of May 26, 1917, which killed 101, left 638 injured and caused $55 million in property damages; and the Tri-State tornado of March 18, 1925, that killed 695, left 2,000 injured and cost $130 million in property damage.

Governor Rauner is grateful that this storm was not as destructive.

“We are very blessed that more people were not hurt. This was a devastating storm,” Rauner told Associated Press writers Michael Tarm and Sophia Tareen after touring some of the damage.

On average, Illinois sees 64 tornadoes per year, mostly during the months of March through May.

The state is near the “Tornado Alley” region of the United States, a nickname given to the Central Plains states for the frequency of tornado occurrences.

In this region warm, moist, low-elevation air from the Gulf of Mexico collides with cool, dry, high-elevation air from the Rocky Mountains. This interaction forces the warm air to rise and the cool air to fall, creating atmospheric instability.

If the opposing winds move at different speeds, the air at the point of collision will start moving around a horizontal axis. Combined with a strong, vertical wind, supercell thunderstorms form, characterized by persistent rotating updrafts.

The continued upward energy shifts the spinning air from a horizontal to a vertical position.

As the air continues to rotate at a faster pace, its height increases, forming a tornado vortex. The vortex has a low pressure core, in which rising air expands, cools and condenses into a funnel cloud. When the funnel cloud touches down on the ground, it becomes a tornado.

On average, most tornadoes have wind speeds that are less than 115 miles per hour. The National Weather Service estimates that wind speeds from the one that hit Illinois on Thursday reached up to 200 miles per hour.

Many people have lost their homes and possessions to the storm. Resident Russell Henson fled the storm with his son and dog after witnessing the tornado destroy a restaurant and move toward his home near the town of Rochelle. Upon returning, he found his house and two barns destroyed.

“We’re alive. I can’t say much more than that,” Henson told the Chicago Tribune.

Southwest Ohio, Indiana and northern Kentucky were under a tornado watch through Friday morning. By Saturday, some of the residents of the affected Illinois communities were allowed back to their homes to assess the damage.

As of Sunday, April 12, police were still searching for missing residents.