  We report on the spectroscopic confirmation of a huge cosmic structure around
the CL0016 cluster at z=0.55. We made wide-field imaging observations of the
surrounding regions of the cluster and identified more than 30 concentrations
of red galaxies near the cluster redshift. The follow-up spectroscopic
observations of the most prominent part of the structure confirmed 14 systems
close to the cluster redshift, roughly half of which have a positive
probability of being bound to the cluster dynamically. We also made an X-ray
follow-up, which detected extended X-ray emissions from 70% of the systems in
the X-ray surveyed region. The observed structure is among the richest ever
observed in the distant Universe. It will be an ideal site for quantifying
environmental variations in the galaxy properties and effects of large-scale
structure on galaxy evolution.
