MENTOR, Ohio- Lake County fire officials determined a foul odor smelled in several lakeshore communities early Thursday resulted from an inversion of Lake Erie’s waters.

Mentor Fire Chief Robert Searles said the department responded to about 20 calls between 5 a.m. and 9 a.m. reporting an natural gas-like odor in the city, near the lakeshore. Similar calls were received by fire departments in Eastlake, Willowick, Willoughby and Mentor-on-the-Lake.

Firefighters and crews from Dominion Energy did not detect natural gas.

Mentor spokesperson Ante Logarusic said it’s the result of lake inversion, which is when strong winds and large waves turn up gunk along the bottom of Lake Erie. Logarusic said it happens once or twice a year.

Ohio Department of Natural Resources spokesperson Jamey Emmert said its biologists determined the foul smell and dead fish along the lakeshore resulted from low oxygen levels in the water. Emmert said it was the result of a natural event called hypoxic upwelling, with heavy waves and winds churning up Lake Erie waters, resulting in lower dissolved oxygen levels. While most fish are fine, forage fish are susceptible to the stress of the event.

“That cold, basically really low oxygen water, was pushed toward shore,” said ODNR biologist Ann Marie Gorman. She said it’s unusual that it happened so close to shore.

“Usually the smell is trapped in the bottom of the lake from that material, but when it moves toward shore that’s when it opens up to the air, and you can smell it,” Gorman said.

Separately, the Mentor Fire Department received a report of a natural gas odor inside Mentor High School around 7:15 a.m. that led to the evacuation of students in the building.

Searles said crews, using a gas sensor, detected a very low natural gas reading in the building, which it determined resulted from a small leak in a line to an HVAC unit. The line was shut off and students returned to all but two classrooms after about a half hour, according to Searles.