A smoldering underground fire on Auahi Street in Kaka­ako on Monday afternoon was caused by welding, according to fire and construction company officials.

A 4-by-6-foot piece of lumber, part of the shoring for underground work to replace sewer lines, caught fire when hot metal slag fell into the hole while workers welded a metal plate, officials said.

Firefighters poured water through holes in the plates until construction workers with heavy equipment removed them so firefighters could put out the fire.

Police closed Keawe Street from Ala Moana Boulevard to Pohu­kaina Street and Auahi Street from Punchbowl to Coral streets as fire crews fought the blaze, which was reported at about 4:30 p.m. It was extinguished about two hours later.

Thick, black smoke poured from underground until firefighters were able to bring the blaze under control.

Construction crews have been working on improving the sewer system in the area for the past several months.