Across two very different continents, scientists are mapping vast systems of ancient tunnels that were not carved by rivers ...

Billionaire entrepreneur Elon Musk's mysterious tweets about tunnels have sparked a flurry of speculation about his next project -- and he apparently did some research last week in D.C. The founder of ...

Deep beneath the hillsides and suburbs of Brazil, there are tunnels so vast and regular that they look like abandoned subway lines. Their walls are smooth, their ceilings arched, and in some cases ...