What lies beneath a city shapes everything built above it. Much of #SanFrancisco sits on land that isn’t really land at all. Large parts of the city were built on artificial fill, meaning dirt, rock, and debris dumped into the bay to create new ground. Mixed into that ground are ancient oyster shells and ships abandoned during the Gold Rush. As the city grew, all of that was buried beneath the streets we walk today, creating an engineering challenge few cities have to solve. Before a building becomes part of the skyline, engineers have to understand what's beneath it. On some sites, that means designing foundations that go more than 200 feet below the surface just to reach stable ground. The most important part of a building is often the one you'll never see.

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