  At very high energies the Earth magnetic field, although in many other
connections conside red rather weak, acts as a strong field on photons and
leptons. This paper discusses the i ntimate connection between this effect and
the corresponding `strong field effects' observ ed at accessible accelerator
energies in aligned single crystals. As such, these effects c onstitute an
experimental verification of the theoretical basis used for simulations of th e
development of electromagnetic showers in magnetic fields, in particular the
geomagnetic field. A short discussion of more general aspects of the shower
development in the fields present at different distance scales is included.
