When lightening strikes the earth how deep does it go? Are buried hydro/phone lines at risk of frying?

Here is some interesting data (emphasis is mine) from tests that Sandia Labs did after the Sago Mine explosion. (Sandia Labs are experts on lightning danger.)

“For measuring propagation of lightning energy from the surface of the earth to the mine cavern 300 feet below, the drive signal was applied to a long wire stretched on the surface. Directly below, inside the mine, an antenna was set up to pick up the transmitted signals. Multiple antenna measurements were made, covering a cross pattern in the mine of about 80 meters by 80 meters. The measurements were compared to analytical models simulating lightning field propagation through the earth.

The data was used to develop transfer functions, a way of understanding how much energy penetrated into the mine based on a surface lightning events. These results were combined with a theoretical lightning strike waveform to determine if voltages get high enough inside the mine to be of concern.

The study concluded that it was highly unlikely that lightning initiated the explosion by traveling along conductors through the mine and into the sealed area. However, electromagnetic energy from a significant lightning event close to the sealed area could travel through the ground at Sago and induce a high voltage into metallic conductors that were left in the sealed section of the mine. A pump cable was left abandoned at the ignition point. This cable was found by MSHA investigators to have many damaged sections that could provide a spark when the cable is subjected to high voltages. A spark is a known ignition source of flammable methane mixtures that can accumulate in sealed sections of mines.”