I was listening to a podcast from the Economist, and I learned that physicists are now using the ocean to look for neutrinos. Previously, they built very expensive large volumes of very clear water & looked for flashes. Now, they are looking only for certain types of sounds that the flashes would have caused. All that is necessary is to filter out every other sound, including whale music. Since sound propagates extremely well in water, the scientists can now utilize entire oceans to look for their neutrinos, dramatically enhancing their ability to detect them. Soooo, set up a neutrino source on one side of the Earth that can be modulated, & utilize the ocean to listen in from the other side of Earth. Since the neutrinos travel at ~c, while the speed of light in fiber optics is currently ~2/3c, and the fiber optics must also follow the Earth's curvature, one could conceivably beat the speed of fiber optics (not on speed, but latency). Of course, if the signal has to travel very far in the water before being detected, this would definitely slow things down, so it would be better to have lots of microphones connected via the best fiber optic cable. The number of bits per second doesn't have to be very high to be valuable -- e.g., talking to submerged submarines or breaking news for stock markets. Of course, speeds in fiber optics can also be improved by using vacuum (or even air) cores instead of glass to get to ~c, but they still can't avoid following the Earth's surface.