A smoldering underground fire Monday afternoon on Auahi Street 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 the sewer lines, caught on fire when a piece of hot metal slag fell into the hole while workers welded metal plate shut on Auahi Street near the Keawe Street intersection, officials said.

Firefighters poured water through holes in the metal plates, before construction workers used heavy equipment to remove the plates so that firefighters could extinguish the blaze.

Police closed Keawe St. from Ala Moana Blvd.  to Pohukaina St. and Auahi St. from Punchbowl Blvd. to Coral St. as fire crews worked to extinguish the fire, which was reported at about 5:30 p.m. and extinguished at about 7 p.m.

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

Construction crews have been working on improvements to the underground sewer system in the area for the the last several months.