An explosion near the intersection of King Street West and Bay Street sent billowing plumes of brown smoke through Toronto’s business district on May 1.

The explosion was due to an overheated hydro vault, CBC News reported, citing Toronto Fire Services. The vault was filled with high-voltage transformers, and smoke erupted from grates along the street when the vault exploded.

The smoke was contained and the surrounding area closed off by about 8 pm local time, according to a tweet from Toronto Fire Services Chief Matthew Pegg. Platoon Chief Kevin Shaw told local reporters that the explosion could have been caused by “dampness, water or an aging hydro vault,” and that authorities were investigating.