[ridgeguy]

Nuclear reactors produce neutrinos and they can be detected [1]. For modulation, maybe vary the reactor power output (low bit rate, to be sure).

People have explored neutrinos for communication with submerged submarines [2].

[1] https://en.wikipedia.org/wiki/Neutrino [2] http://www.physics.ucla.edu/~hauser/neutrino_communication_p...