There is a reasonable basis f or post ul ating t he existenc e of a useful waveg uide deep in the earth's crust, of t he order of 2 t o 20 km below the s urface. Its di electric is basem ent I'Oc], of very low condu ctivity. Its upper boundary is formed by the conductive layers ncar the surface. Its lower boundary is formed by a h igh-temperature condu ctive layer far below the surface, termed the "thermal ionosphere" by analogy to the well-known "radiation ionosphere" far above the surface.The electrical conductivity of th e basem.ent rock has not been explored. An example based on reasonable estimates indicates that transmission at 1.5 kc/s might be possible for a distance of the order of 1500 km.This waveguide is located under land and sea over t he entire surface of th e earth. It may be useful for rad io transmission from t he s hore to a submarine on the fl oor of t he ocean. The sending antenna might be a long conductor in a dri ll hole deep in the basement rock ; the receivi ng antenna might be a vertical loop in t he water .