We've mentioned before how great it would be if the future of aviation included airships (blimps). The main problem with them, though, is that they're slow. But what if you could keep the advantages of an airship--minimal fuel use--with higher speed? And while you're at it, have the higher speed come from something that reduced fuel use even further? Well, as GizMag writes, Robert D. Hunt has thought of just that. Though GizMag doesn't describe it well, nor does the inventor on his website--he seems to think this is some kind of perpetual-motion machine--the basic principle is a smart one: the plane uses buoyancy to lift itself up high, then acts like a glider, trading its height (potential energy) for speed (kinetic energy). The only power use would be what's required for compression and expansion of the ballast gas (or vacuum) for buoyancy. These power needs would be a tiny fraction of what's now used for aircraft, and should even be much smaller than what's used for airships, because airships still have to push themselves around horizontally using propellers. What little power is used on a glider-blimp (or "gravity-powered aircraft", as Hunt calls it) might not even be provided by burning fuel, but by onboard batteries, making it truly a fuel-less plane, as Hunt claims. It would also certainly be quieter than any other powered aircraft technology.
Note that this is still just a concept--i.e. vaporware--but it's an interesting idea, that might have a future.
The concept is used for scientific ocean probes and the idea isn't terribly new. The density of water gives you real flexibility for picking the right materials that make this happen. Doing it in a lower density environment would be much more difficult