Think the subways are dirty? Imagine the MTA facilities you can’t see.

Inspections of five transit boiler rooms revealed “disturbing and extreme structural and safety concerns,” MTA Inspector General Carolyn Pokorny said Monday — including crumbling, rat-infested boiler rooms and hazardous water conditions that put transit workers in danger of electrocution.

Pokorny’s office launched the inquiry in response to complaints concerning the boiler room at the East New York Yard, where IG staff found fire hazards and tripping hazards, as well as violations related to emergency egress.

They also found flooding in the Jerome Avenue boiler room so high that “workers had to walk on makeshift wood planks” to avoid standing in water — including while working on electrical equipment.

The IG called the findings “alarming.”

“It is unacceptable to expect workers to report to work in what are essentially preventable, extremely hazardous conditions,” Pokorny said in a statement.

At 205th Street, investigators found structural decay impacting both the safety of workers and the integrity of the boiler — which was located in a dilapidated, rat-infested “temporary” structure in place for over a decade. At 207th Street, workers had installed tarps immediately beneath the ceiling to protect electrical equipment, and themselves, from leaking asbestos-filled water.

Pokorny blamed the years of neglect by MTA leadership and personnel.

“These concerns were known to management … but the problems were not addressed,” Pokorny wrote in the report summary sent to the MTA on Friday.

The findings — which precede a full audit to be released at a later date — provide ammunition for transit labor leaders, who have griped about underground working conditions as management seeks to increase employee health care contributions and cut back on overtime.

“Now you know why transit workers are so pissed off,” said TWU Local 100 president Tony Utano, whose union is in the thick of heated contract negotiations with MTA management. “We work in horrendous conditions — exposed wires, asbestos, swarms of rats.”

In a statement, MTA spokesperson Meredith Daniels pointed to steps the agency has taken or plans to take at each of the workplaces visited by the IG and said a “full system-wide inspection” is underway.

“The safety of the MTA’s workforce is our top priority as is the proper maintenance of all our assets,” Daniels said. The MTA “corrects defects during the course of routine inspections and maintenance at these facilities is already underway.”