Before drone strikes and cyberattacks, long-range missiles and nuclear annihilation, wars were fought with cannonballs and other solid orbs of destruction. That mode of attack might help explain why more than 16 million bricks were used to build Fort Jefferson and it has still never been completed. Construction of the "Guardian of the Gulf of Mexico" began in 1846; officials called it off in 1875 due to concerns that the sheer weight of all those bricks was too stressful for the tiny island and its water system, according to the National Park Service. Located about 70 miles west of Key West in Dry Tortugas National Park, the imposing structure is little more than a subtropical ghost town these days. More than 160 years of storms and salty sea winds have taken their toll, but if you can get out to the small remote islands, you'll stroll around a one-of-a-kind artifact that would never be deemed feasible in these modern times.