  Earth is constantly struck by radiation coming from the interstellar medium.
The very low energy end of the spectrum is shielded by the geomagnetic field
but charged particles with energies higher than the geomagnetic cutoff will
penetrate the atmosphere and are likely to interact, giving rise to secondary
particles. Some astrophysical events, such as gamma ray bursts and supernovae,
when happening at short distances, may affect the planet's biosphere due to the
temporary enhanced radiation flux. Muons are abundantly produced by high energy
cosmic rays in the Earth's atmosphere. These particles, due to their low cross
section, are able to penetrate deep underground and underwater, with the
possibility of affecting biological niches normally considered shielded from
radiation. We investigate the interaction of muons produced by high energy
cosmic rays on Earth's atmosphere using the Geant4 toolkit. We analyze
penetration power in water and crust and also the interaction effects within
bacteria-like material according to particle type and energy, and notice the
possibility of off-track damage due to secondary particles.
