The search for Magnetic Monopoles (MMs) in the penetrating cosmic radiation remains one of the main items of non-accelerator particle astrophysics.

Grand Unified Theories (GUT) of electroweak and strong interactions predict the existence of superheavy magnetic monopoles with masses larger than 10¹⁶ GeV. They would have been produced at the end of the GUT epoch, at a mass scale ~ 10¹⁴ Gev and the cosmic time of ~10^-34 s. Such monopoles cannot be produced with existing accelerators, nor with any foreseen for the future. Lower mass monopoles (10⁶-10¹⁰ GeV) originated in the early universe have been proposed by many authors.

With SLIM experiment we are searching for relatively light MMs with an array of 400 m² of passive nuclear track detectors deployed at the high altitude Chacaltaya Lab (5230 m high above sea level). In >4 years of operation we should be able to reach a sensitivity at the level of Parker bound, i. e. 10^-15 cm^-2 s^-1 sr^-1. Byproducts of this MM search are the searches for relatively light nuclearites and Q-balls.

An exposure at high altitude laboratory would allow to search for MMs of lower masses, higher magnetic charges and lower velocities. The high altitude exposure would allow detection of the above mentioned particles even if they had strong interaction cross sections which could prevent them from reaching the earth surface.