Because the PIV rating of LEDS will not withstand anything even close to full mains voltage. They are not constructed as rectifiers, even though they are technically diodes. This datasheet shows the max PIV for a high performance white LED to be a mere 5V! A typical 1N4007 rectifier diode is rated for 1000V PIV DC.

I've been in my new house for 6 years now, and I've replaced 4 incandescent bulbs, I think. These lights are used daily, and many of the ones still burning are turned on/off many times a day (bathroom lights).

I have an issue with LED and CFL that many don't - the electronics inside them (unless very well designed) make an absolute hash of radio bands from DC to about 30 MHz. As an amateur radio operator, I can't have that going on in my house (and preferably those houses around me).

I know the US Navy has performed experiments using lasers for submarine-to-surface comms - no idea on how well that went.

How much does a 300km spool of fiber optic cable weigh again? :-)

I wonder if sound would provide a very low bandwidth channel? If you can get good coupling into the ice ot should transmit sound relatively well.

... SLF radio repeaters ...

Why try to brute force RF down at the DC level? Why not head to the other end of the spectrum and use lasers? If that water is relatively pure and there are few bubbles in the ice, I think lasers would win the size/weight to comm distance race.

BTW I love the idea of a modulated neutrino beam, except how much mass would it take to even modulate it enough to be detected?

No current power generation use. It's used for medical lung imaging today.

In a word, Helium3.

But "He Who Controls the Spice Controls the Universe!"

You left out "Heavens to Betsy, No!!!".

OTOH the Soviets had a serious love affair with RTGs ...

Nevermind the RTGs - I'm much more concerned with the nasty fission reactors they've put up there. The US put one up, too.

... they buried him in a photon torpedo-shaped casket, wearing a black robe.

Apple did the same thing with OSX 10.10 / Yosemite. The 'new' icons are flat and just plain nasty. I assume everyone wants to 'streamline the user experience' across phones, tablets, watches, and real computers, but I think pandering to the lowest common denominator is just a bad idea.

... using how much bandwidth and on what frequencies? Even at 100m distance, this may be a non-starter if the attenuation of walls and other normal, everyday things kills the signal.

As another commented, use kermit or zmodem. There are also LPT-to-LPT methods, but IIRC that requires a special cable.

What interface is the HD in the laptop? You could likely rip it out and put it in an old USB external drive enclosure and mount it via USB.

Of course you're correct - duh on my part.

Note that OUR government has it's nose in our copiers to this day to prevent copying anything exhibiting the Orion Constellation like most world currency. Check the backs of recent US currency - the pattern is there.