Emergency crews evacuated a business inside an underground storage cave in Kansas City on Friday morning after high carbon monoxide levels were found, and four people were taken to hospitals.

The Kansas City Fire Department was called around 8:30 a.m. to a report of an unconscious person at the business near NE Underground Drive and Missouri 210 Highway. Crews treated nearly two dozen people and ordered the evacuation as they checked the air inside the underground complex.

Firefighters later determined that the problem was carbon monoxide, not the medical emergency that first brought them in. They stayed on scene for several hours to keep monitoring air quality in the cave, a reminder of how quickly a call can turn into a hazardous materials response when invisible gas is involved.

Crews cleared the scene shortly before 11 a.m., and people were allowed to go back to work inside the cave. The fire department did not say what caused the carbon monoxide buildup, leaving the question of how the gas accumulated unanswered even after the all-clear.