How are bridges like the San Fran build in the water?

Question

How do they manage to swim all the way to the bottom to build the foot of the bridges in that deep cold water long ago?

Answer 1

I  heard the send them down in boxes and stuff..

Answer 2

They build what's called a coffer dam around the area where the concrete foundation will be on the bottom and then build a form and then pour the concrete.  Then building of the tower begins.

The coffer dam is built by pile-driving sections of corrugated steel into the bottom.