A usual first principle in planning radioastronomy observations is that monitoring must be carried out well above the ionospheric plasma cutoff frequency (~5 MHz). Before space probes existed, radioastronomy was almost entirely done above 6 MHz, and this value is considered a practical lower limit by most radioastronomers. Furthermore, daytime ionization (especially D-layer formation) places additional constraints on wave propagation, and waves of frequency below 10-20 MHz suffer additional significant attenuation. More careful calculations of wave propagation through the earth's ionosphere suggest that for certain conditions (primarily the presence of a magnetic field) there may be a transmission window well below this assumed limit. Indeed, for receiving extraterrestrial radiation below the ionospheric plasma cutoff frequency, a choice of VLF frequency appears optimal to minimize loss. The calculation, experimental validation, and conclusions are presented here. This work demonstrates the possibility of VLF transmission through the ionosphere and various subsequent material barriers. Implications include development of a new robust communications channel, communications with submerged or subterranean receivers / instruments on or offworld, and a new approach to CETI.

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