The $Lambda$CDM cosmological model successfully reproduces many aspects of the galaxy and structure formation of the universe. However, the growth of large-scale structures (LSSs) in the early universe is not well tested yet with observational data. Here, we have utilized wide and deep optical--near-infrared data in order to search for distant galaxy clusters and superclusters ($0.8<z<1.2$). From the spectroscopic observation with the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the Magellan telescope, three massive clusters at $zsim$0.91 are confirmed in the SSA22 field. Interestingly, all of them have similar redshifts within $Delta zsim$0.01 with velocity dispersions ranging from 470 to 1300 km s$^{-1}$. Moreover, as the maximum separation is $sim$15 Mpc, they compose a supercluster at $zsim$0.91, meaning that this is one of the most massive superclusters at this redshift to date. The galaxy density map implies that the confirmed clusters are embedded in a larger structure stretching over $sim$100 Mpc. $Lambda$CDM models predict about one supercluster like this in our surveyed volume, consistent with our finding so far. However, there are more supercluster candidates in this field, suggesting that additional studies are required to determine if the $Lambda$CDM cosmological model can successfully reproduce the LSSs at high redshift.
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