It's a system that was cutting-edge when it was built nearly a century ago - and remains so cutting-edge, it is the only one of its kind in the entire country.
It's the city's auxiliary water supply system, a network of storage systems and pipes that allows firefighters to access streams of incredibly high-pressure water to battle major blazes like one that could occur following a devastating earthquake.
"A lot of people call it the architectural underground marvel of San Francisco," said Michael Thompson, assistant deputy fire chief. "It's like if there was an underground Golden Gate Bridge, this would be it."It was the brainchild of Dennis T. Sullivan, the city's fire chief in the early 1900s. The city had already burned to the ground five times before the 1906 earthquake and fire, and insurance companies were drastically raising their premiums for property owners because of the fire danger - magnified by the city's topography, winds and high density.
In 1903, Sullivan proposed an underground emergency water supply system that would store vast amounts of water and use gravity to deliver it in high-pressure streams around the city.
The idea was that because San Francisco is largely surrounded by water, it can't rely on adjacent cities to send water in an emergency and should store its own.
Pipes that deliver water from Hetch Hetchy for both the city's regular tap and firefighting uses, as well as its emergency water system, cross fault lines and could be jeopardized during a quake. (That system's seismic retrofitting is already under way.)
But like many City Hall proposals, it took years to come to fruition - during which the April 18, 1906 earthquake cracked water pipes around the city. San Francisco burned for three days, destroying 22,000 buildings and killing as many as 3,000 people.
"The 1906 earthquake caused a great deal of damage, but people often forget that it was the days of fire that followed that truly destroyed much of the city," said Mayor Gavin Newsom, who sponsored the bond measure along with Board of Supervisors President David Chiu.
"This system is worth its weight in gold, but it needs to be maintained," she said.