Operation of Detroit's trash incinerator, the largest in the nation, was halted last month, less than a year after it was put into service. The huge plant, which was designed to change waste into energy, had been hailed as an innovative means of putting trash to good use. However, soon after the plan began operating, unhealthy levels of mercury began to be detected in the air around the city. It was soon discovered that the culprit was the new incinerator. Entire neighborhoods were threatened by the toxins being discharged into the air. Environmental officials ordered the plant to be shut down last week. They say it is uncertain when, or even whether, the plant will reopen. According to the passage, why was the plant shut down?