Alex Coburn, a staff archaeologist at the engineering and environmental services firm Kleinfelder, was monitoring construction near the firebreak established to stop the fires that followed the earthquake when his team found the top of a staircase. They followed the steps underground to a door that opened into a cellar that had apparently remained untouched for more than a century.

The stairs were still functional, and later excavations turned up bottles, cans, and other household items. The discovery offers a rare physical connection to the city as it existed before the catastrophic 1906 earthquake and fire.

Coburn's work involves watching over construction projects for archaeological resources that might be disturbed during excavation. Such finds can range from remnants of old buildings to Indigenous cultural resources. When Native American artifacts or other cultural resources are encountered, the associated tribe is contacted to help determine the appropriate next steps.

San Francisco's underground is a trove of such material. Past projects have turned up buried ships, sometimes used as landfill by early settlers, as well as evidence of Ohlone village sites and former cemeteries. During the Van Ness Improvement Project, which began in 2016, crews encountered buried brick and concrete structures, abandoned utility infrastructure, sand deposits, and archaeological materials. An investigation at 49 Van Ness between 2017 and 2018 uncovered evidence spanning centuries, including Indigenous-era environmental conditions, 19th-century settlement, and remnants of the 1906 earthquake and fire.

In 2012, construction crews digging near Hyde and Fulton streets uncovered the foundations of the city's original City Hall. The French Second Empire-style building, which took 25 years to construct and opened in 1897, collapsed during the 1906 earthquake. Its dome and ruined framework became some of the most recognizable images of the disaster. The newly found foundations were documented by archaeologists before being buried again.

The latest discovery is a reminder of how much of the city's former architecture remains beneath its modern streets. Journalist and architectural enthusiast Daniel Coughlin recently created detailed renderings imagining what San Francisco might look like today had the 1906 earthquake never happened, bringing back the original Palace Hotel, Nob Hill, Old City Hall, Chinatown, and the grand buildings of Market Street. Coughlin described his reimagining of Nob Hill as particularly striking, calling it “a majestic skyline that has largely vanished from people's memory,” filled with elaborate mansions belonging to some of California's wealthiest families, soaring church spires, and the distinctive domes of Temple Emanu-El.

For local residents, the sealed basement is a tangible reminder that the ground beneath the city's busy corridors still holds the everyday life of a San Francisco that disappeared in 1906.