Here’s a good trick: The gravityplane, brainchild of inventor Robert Hunter, will be able to change its density from lighter-than-air to heavier-than-air. The aircraft, still in development, will be similar to a submarine that changes its buoyancy, a form of gravity, to float on the surface of the sea or cruise 300 ft below it. If the design pans out, the plane won’t need any fossil fuel and will have a virtually unlimited range.