Gallons of foam unexpectedly started oozing out of a street in China, shocking commuters and passersby.

The substance, which was leaking out of a street in the city of Nanjing, covered an area of 40-50 square meters, according to a translation of news outlet Longhoo.

Officials are unsure what the substance might be and offer only that it could possibly be a byproduct of subway construction in the area.

Firefighters and police quickly cordoned off the area and washed the foam away.