Human ingenuity.

The blogger from Rands in Repose gives us some insight into building the Brooklyn Bridge:

"With the caisson on the riverbed, it’s time to push it another 45 feet into the riverbed in search of bedrock. Workers did this through the continued application of stone to the top while workers in the caisson dug out the riverbed with shovels, buckets, and, when necessary, dynamite. There was nothing resembling an electrical grid, so there was nothing resembling modern lighting in this watertight pine-tarred box, which was slowly descending through the floor of the East River. There were no jack hammers, so when they hit rock, they used small amounts of dynamite to crack these rocks. In a pine-tarred box, at the bottom of a river, mostly in a very wet dark.

And when the caisson finally hit bedrock 45 underground, they had to do it all over again for the New York tower. 30 feet deeper."