  All galaxies once passed through a hyperluminous quasar phase powered by
accretion onto a supermassive black hole. But because these episodes are brief,
quasars are rare objects typically separated by cosmological distances. In a
survey for Lyman-alpha emission at redshift z ~ 2, we discovered a physical
association of four quasars embedded in a giant nebula. Located within a
substantial overdensity of galaxies, this system is probably the progenitor of
a massive galaxy cluster. The chance probability of finding a quadruple quasar
is estimated to be ~10^-7, implying a physical connection between Lyman-alpha
nebulae and the locations of rare protoclusters. Our findings imply that the
most massive structures in the distant universe have a tremendous supply (~
10^11 solar masses) of cool dense (volume density ~1 cm^-3) gas, which is in
conflict with current cosmological simulations.
