Neutrino

Neutrinos were subatomic particles that could pass through most matter. They could be detected with neutrino detectors, which were vats of cleaning fluid with sensors inside. In the rare instances when a neutrino interacted with an electron in the fluid, the sensors would detect it.

The Arkive communicated with neutrinos. In order to understand it, the Second Doctor had to create a neutrino detector in the ice of Mnemosyne. (PROSE: The Wheel of Ice)