I did not know that yesterday!

In the old days, building bridges required the sinking of many large rocks. Not the most elegant construction technique, maybe, but certainly quite effective. The Romans built bridges this way over 2,000 years ago that are still in use today, but most modern foundations for large bridges begin their lives as huge floating tanks.

During the construction of the Brooklyn Bridge, laborers worked in pneumatic caissons, or huge airtight cylinders full of compressed air. Conditions inside the caissons were notoriously miserable; twenty men died from fires, explosions, and something called "Caisson Disease," which we now know as the bends. No-one knew much about the effect of air pressure on the human body back then, and the designer, Washington Roebling, was paralyzed for life.

In the 1930s, workers on the San Francisco Bay Bridge used giant submersible caissons as actual bridge foundations. Initially buoyant, these huge steel contraptions were towed out to the bay, then gradually filled with concrete and water, and sunk. By that time diving technology had improved, so individual divers, or "sand hogs," assisted in the process.

The same method is used today -- huge metal caissons weighing up to 30,000 tons are floated out to a designated spot and then sunk. For example, the foundations for the Tacoma Narrows Bridge were pumped with 12,000 tons of sea water.