[geofft]

In some sense, that's "just" radio. An antenna is a wire... it's sending a signal and picking it up from a very nearby antenna. Radio transmissions are all AC.

(Arguably, the real insight here is that the very existence of radio is impressive / unintuitive.)

[y04nn]

The gap on the line acts more as a capacitor than an antenna, AC pass through capacitors, but DC does not.

[Johnythree]

It's a differential signal using a center-tapped transformer to drive the signal. It's actually using three wires if you count the ground.

This means it's symmetrical about ground and thus cancels it's own radiated field (eg doesn't interfere with adjacent lines).

But with one side open there is still a path for AC, which is through the other side and using the ground return.