Interesting holes you have us falling into Max.
https://www.sfgate.com/bayarea/article/S-F-sewer-work-must-navigate-around-buried-ships-4677060.php

On the surface, San Francisco is in a growth spurt, with new construction dotting the landscape and tech companies sprouting like weeds in South of Market lofts.

But below the streets lies an older San Francisco, one filled with buried ships, abandoned tunnels and aging sewer pipes on the brink of failure.

"They don't even make this stuff anymore," said Alan Wong, holding a piece of square rebar that he had just pulled from the side of an open sewer pit 12 feet below the surface of Drumm Street.

"jackhammers working to excavate the foundation of a building buried beneath the street in front of the Hyatt Regency Hotel".

In a city with nearly 1,000 miles of sewer lines, navigating a subterranean jungle of buried relics from San Francisco's past.

"We knew the bay was higher, but what we didn't know was that this whole area was built on fill," she said. "We never know what we're going to find down there."

Hidden from prying eyes for more than a century. They likely would have remained hidden longer if not for the 1906 Great Earthquake and Fire. Among other acts of devastation, the fire obliterated Chinatown and uncovered its many deep dwelling places.