Before the bridge, the only way to go north from San Francisco to Marin County was by ferryboat. The trip by ferry took 20 minutes to complete. Before the bridge was built, San Francisco was the biggest American city still served mostly by ferries. Some experts did not think that people could build a bridge because the water currents between Marin and San Francisco were very strong. Also, the water was very deep in the middle - 102 m - and there was a lot of wind and fog that could make building a bridge difficult.