Mark J Schilling

For a region best known for six million acres of forest, the Adirondacks concentrate a remarkable amount of their character onto a handful of short commercial streets. Adirondack Park contains around 105 towns and villages, and the handsomest of their main streets work much the same way, with galleries, theaters, and waterfront access compressed into a few walkable blocks under visible mountains. What makes them pretty is mostly what was never torn down.

Roughly a million people crowded into ancient Rome at its height. Chang'an held nearly as many along the eastern end of the Silk Road. Engineers in these cities built aqueducts and drainage systems to keep such numbers fed and watered. Planners laid out markets and civic districts on measured grids. The cities in this list rank among the largest the ancient world ever produced. No censuses survive for most of them.

More than 3 million shipwrecks are lying on the floor of the ocean and other deep bodies of water. That's the estimate from scientists, archaeologists, and historians, and the number is based on available research and data, but the truth is, no one knows for certain. Less than 1% of known shipwrecks have been mapped, located, or explored by humans. For as long as humans have been venturing out onto deep water in boats, there have been wrecks.