SYLMAR >> First came the smell of gas, then the blast that killed 17 miners deep beneath the San Fernando Valley neighborhood of Sylmar.

On June 24, 1971, the Sylmar tunnel explosion occurred.

It was the worst tunnel disaster in California history, leaving one survivor. Ralph Brissette, who had been standing behind a mine locomotive, was trapped for hours before two courageous rescuers braved the tunnel to find him.

“It’s important to remember this event because of the families lost. They died trying to bring water to Los Angeles,” said Peter Rosenwald, 71, of Seal Beach, who had advocated for a Sylmar tunnel blast memorial.

Their lives have not been forgotten, and they will forever be remembered on a memorial.

“The memorial now commemorates these brave men and the many firefighters who tried to rescue them.”

The Sylmar tunnel explosion occurred deep underground a day after a flash fire at its entrance injured four men.

That’s when 17 elite miners and Metropolitan Water District Inspector Louis L. Richardson were boring a 5.5-mile water tunnel to ferry state water from Castaic to points east and suddenly smelled gas — lots of it.

Then came the boom, likely caused by subterranean gas jarred loose by the Sylmar earthquake some four months earlier. Nine miners were killed outright, eight others soon suffocated from the smoke and dust.

The disaster would spur the toughest mining and tunnel regulations in the state and nation, and a year-long criminal trial against the contractor, resulting in record fines and civil damages.

Nineteen Los Angeles city firefighters were awarded the Medal of Valor, a record for a single incident.

The disaster also sparked a 54-week criminal trial against its contractor — a record for the City Attorney’s Office.

The result: some of the highest municipal fines and greatest civil damage awards of its time.

After the longest Municipal Court trial in U.S. history, Lockheed Shipbuilding & Construction Co., a subsidiary of Lockheed Aircraft, was found guilty of gross negligence and violating state safety laws. The company was fined $106,250 and forced to pay $9.3 million in civil judgments.

In 2013, the Metropolitan Water District of Southern California unveiled a long-awaited monument to the victims and their emergency responders outside its downtown L.A. district headquarters.

“They were the best of the best,” said Brissette, now 78, of Culver City, of his lost friends during the memorial commemoration.

Since the disaster, he has devoted his entire life to their memory.