The record for the tallest building in the world currently belongs to the Burj Khalifa in Dubai, but architects in China and Saudi Arabia are hoping to top it. Nate Berg wonders if we'll ever hit a limit. He spoke to William Baker, a structural engineer who worked on the Burj Khalifa:

A building taller than a mountain seems preposterous. But according to Baker, it's entirely possible. "You could conceivably go higher than the highest mountain, as long as you kept spreading a wider and wider base," Baker says. Theoretically, then, a building could be built at least as tall as 8,849 meters, one meter taller than Mount Everest. The base of that mountain, according to these theoretical calculations, is about 4,100 square kilometers – a huge footprint for a building, even one with a hollow core. 

(Image of a 1990s-era concept for a two-and-a-half-mile supertower in Tokyo with a hollow core similar to the Eiffel Tower's.)