  We report the discovery of primeval large-scale structures (LSSs) including
two proto-clusters in a forming phase at z=5.7. We carried out extensive deep
narrow-band imaging in the 1 deg^2 sky of the Subaru/XMM-Newton Deep Field, and
obtained a cosmic map of 515 Ly alpha emitters (LAEs) in a volume with a
transverse dimension of 180 Mpc x 180 Mpc and a depth of ~40 Mpc in comoving
units. This cosmic map shows filamentary LSSs, including clusters and
surrounding 10-40 Mpc scale voids, similar to the present-day LSSs. Our
spectroscopic follow-up observations identify overdense regions in which two
dense clumps of LAEs with a sphere of 1-Mpc diameter in physical units are
included. These clumps show about 130 times higher star-formation rate density,
mainly due to a large overdensity, ~80, of LAEs. These clumps would be clusters
in a formation phase involving a burst of galaxy formation.
