  We report the detection of a significant ionospheric disturbance in the
D-region of Earth's ionosphere which was associated with the massive gamma-ray
burst GRB 221009A that occurred on October 9 2022. We identified the
disturbance over northern Europe - a result of the increased ionisation by X-
and gamma-ray emission from the GRB - using very low frequency (VLF) radio
waves as a probe of the D-region. These observations demonstrate that an
extra-galactic GRB can have a significant impact on the terrestrial ionosphere
and illustrates that the Earth's ionosphere can be used as a giant X- and
gamma-ray detector. Indeed, these observations may provide insights into the
impacts of GRBs on the ionospheres of planets in our solar system and beyond.
