MEXICO CITY (AP) — Travelers on the Mexico City subway system often blame authorities for broken-down escalators at subway stops, but Metro officials have another explanation: vast amounts of pee.

Somehow, urine is penetrating and corroding the drive wheels and mechanisms of the escalators that carry riders up from underground stations.

In a list published Tuesday, the Metro system listed “corrosion due to urine” as one of the top five causes of escalator breakdowns.

Fermin Ramirez, the system’s assistant manager for rails and facilities, said riders appear to be urinating on escalators at off-peak hours and lightly used stations, “even though it seems hard to believe.”

“When we open up escalators for maintenance, there is always urine,” Ramirez said. Most stations have no public bathroom facilities, a fact Twitter users were quick to point out, noting there are not even any pay toilets.

Of the system’s 467 escalators, 22 are out of service on any given day.

The biggest problem, subway authorities admit, is that the many escalators are old, or have been damaged by rough use.

The city plans to replace about 55 escalators over the next two years.

With over 1.6 billion rides per year, the Mexico City subway is considered the eighth largest in the world by some measures, and one of the cheapest: A 25-cent ticket will get you a single ride to any destination on the 140-mile (226-kilometer) system.

Just remember to use the bathroom before setting out.