The Richmond Municipal Natatorium at 1 East Richmond Avenue — a 1926 indoor pool once left to swim-at-your-own-risk and marked for the wrecking ball — reopens its doors for a centennial party Saturday, 15 years after volunteers pulled it back.

Stand on the deck at 1 East Richmond Avenue and the first thing you notice is the light. More than 170 windows line the two-story masonry hall — 132 of them built to crank open — and the ceiling above the water is a bare steel truss. Today, Saturday, the city is throwing the room a birthday party.

The Richmond Municipal Natatorium, which nobody calls that — it's the Plunge — is 100 years old (external source, opens in a new tab). The city has blocked out the whole day: a dedication with speeches from 10 to 11 a.m., in-water demonstrations from the pool's programs until 12:30, drop-in lap swim from 1 to 3, and a family recreation swim from 3:30 to 5:30 that runs $1 a head — online sign-up only, capped at 150.

Getting the doors open at all took two decades of argument. A 1926 bond built the place, and it opened that year outsized on purpose: a single basin twice the footprint of an Olympic pool, balconies for spectators, water pumped in salty from the bay the way San Francisco's vanished Sutro Baths once did. Then it aged badly. Years of skipped upkeep split the clay-tile walls, the 1989 Loma Prieta quake widened the cracks, and when voters turned down a reinforcement bond in 1997 the city decided it could not carry a retrofit. People swam anyway, past posted "swim at your own risk" warnings, until Richmond shut the building in August 2001.

Demolition was the next line item. Neighbors pushed back, and Richmond Friends of Recreation — a rec-boosting nonprofit that had been around since 1979 — organized the rescue. Its "Save the Richmond Plunge Trust" pulled together more than $154,000 in cash (external source, opens in a new tab) and another $40,000 in donated materials, and the city agreed to a public-private partnership (external source, opens in a new tab) to fund the retrofit. Because the Plunge sits inside the Point Richmond Historic District on the National Register, every fix had to leave the landmark intact. Berkeley architect Todd Jersey lifted part of the roof to add the windows and donated hundreds of hours; work began in August 2008 and the pool reopened in 2010 after nine dry years.

The old single basin is now two — one side for laps, one for play. Among the regulars KQED found at the pool this month (external source, opens in a new tab) was Otis Mobley, 62, who swam here as a boy in the 1970s and had come back to watch his 12-year-old granddaughter, Naomi, pass her deep-end test. Walk past East Richmond Avenue tomorrow and the windows will still be cranked open to the light.