An underground steam pipe burst on Grant Avenue Sunday afternoon, shutting down the block between Geary and Post as San Francisco firefighters, Public Works staff, and PG&E responded to the scene.

The pipe burst sometime after noon, prompting an AlertSF message about "fire activity" on the block of Grant between Geary and Post.

The public was advised to avoid the area.

Photos from the San Francisco Fire Department showed a cloud of steam, that could be mistaken for smoke, rising from the ground and filling the street.

An underground network of steam pipes that is nearly a century old is responsible for heating and cooling many downtown buildings, as well as City Hall and the Moscone Center.

As Secret SF explained recently, when you see steam rising out of grates around downtown SF, it's likely coming from this system — and today's steam pipe rupture was likely part of the system as well.

A company called Cordia Energy runs the system, which spans about two square miles in and around downtown, and services over 180 customers.

In the midst of the drought in 2015, KPIX did a story about how this steam pipe network is using SF drinking water — about 250,000 gallons per day — to operate the heating and cooling system. At the time, the company managing it was called NRG.

The map below of the steam pipe network suggests that Grant Avenue may be a prime piece of the distribution network, but no steam customers appear to be on that street.