  Deep underground laboratories are the only places where the extremely low
background radiation level required for most experiments looking for rare
events in physics and astroparticle physics can be achieved. Underground sites
are also the most suitable location for very low background gamma-ray
spectrometers, able to assay trace radioactive contaminants. Many operational
infrastructures are already available worldwide for science, differing for
depth, dimension and rock characteristics. Other underground sites are emerging
as potential new laboratories. In this paper the European underground sites are
reviewed, giving a particular emphasis on their relative strength and
complementarity. A coordination and integration effort among the European Union
underground infrastructures was initiated by the EU-funded ILIAS project and
proved to be very effective.
