An engineering feat first attempted in Babylonian times, tunnels are an integral part of 20th century society. Yet they remain among the most dangerous projects ever undertaken. Here are the stories of the world's great tunnels, of the genius of men andthe marvels of machinery. From Brunel and Greathead, who developed an innovative shield which allowed them to burrow under the Thames, to Clifford Holland, who overcame nature and technology to carve his 8,463 foot path below the Hudson, few of the great architects lived to see their dreams completed. Perishing in greater numbers were 'sandhogs,' workers who labored beneath unforgiving waters to forge these indispensable subterranean arteries. See why that danger remains today with a look at the construction of New York's new water tunnel, inching forward even as the death toll climbs.