What: A 10,000-year storage facility for radioactive nuclear waste
Where: yucca mountain, nevada
Cost: $58 billion
Crux: Approximately 60 miles of access and emplacement tunnels to house stainless-steel waste casks. A robotic transport system of gantries, cranes and railcars.

What: An "elevator" that carries heavy loads into space
Where: equatorial pacific ocean
Cost: $10 billion
Crux: A 62,000-mile cable -- one end "anchored" in space and the other attached to a platform in the Pacific -- that acts as a rail for laser-powered lifters carrying up to 5 tons.

What: The largest man-made offshore islands
Where: dubai, united arab emirates
Cost: $3.5 billion

Crux: Two islands built in the shape of giant palm trees. The "trunks" are 5 miles long, each topped by 17 "fronds" up to 330 feet long. A third island group is shaped like a flat map of the globe. Materials: 4.2 billion cubic feet of dredged sand and 50 million tons of rock.