A snowlike foam fire extinguishing agent covered 500 meters of the Tokyo Bay Aqua-Line tunnel, an expressway that connects Kanagawa and Chiba prefectures across the Tokyo Bay.

According to East Nippon Expressway Company, fire extinguishers along the upper half of the 9.6-kilometer tunnel malfunctioned on September 18 just past 9AM, spraying foam and water inside.

At the time, workers were testing the system during renovation work of emergency equipment.

The extinguishing agent sprayed for 9 minutes before workers were able to turn it off.

The scene of the incident was 7.3 kilometers from Kanagawa Prefecture, and the road heading towards the prefecture was closed for 1 hour and 40 minutes while it was being cleaned.

Officials are investigating the cause of the false alarm.