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This paper discusses four methods of electrical communication within the earth's crust. Mode I may be described as similar to propagation over the ground, except that the rock dielectric substitutes for air as the propagation medium and the under surface of the overburden takes the place of the earth's surface. Mode II results when the of the propagating medium is small so that the antennas must be modified, which also results in a modification of the propagation formulas. Two other modes are considered for conditions where the depth to the good dielectric basement rock is prohibitive. Mode III is indicated when the overburden is a highly conducting layer. It makes use of "dc signaling" with vertical antennas extending below the surface layer. Mode IV is indicated when the overburden is so thin that it does not provide an effective conducting plane. It makes use of "dc signaling" with the antennas consisting of a pair of vertical conductors insulated from the thin layer of higher conductivity, but making contact with the medium of intermediate conductivity, hence, generating a horizontally polarized wave. Distances and information rates are given for these modes under various propagation conditions.