| Abstract: We present a new cosmological zoom-in simulation, where the zoomed region consists of two massive halos, $M_{rm vir}gsim 5times 10^{12}msun$ at $zgsim 2$, and a giant, $sim Mpc$ long filament connecting them. Using this simulation, we study the evolution of the intergalactic medium (IGM) in between the two main halos at redshifts $5>z>3$ at unprecedented resolution. At $5gsim z gsim 3$, the two halos are found to lie in a large intergalactic sheet, or ``pancake", consisting of multiple co-planar dense filaments along which nearly all halos with $M_{rm vir}>10^9msun$ are located. These filaments merge within the sheet, forming the single large filament connecting the two halos at $zgsim 2$. This sheet collapses at $zgsim 5$ from the merger of two smaller super-galactic sheets. The strong shock generated by this merger leads to thermal instabilities in the post-shock region, and to a shattering of the sheet resulting in $lsim kpc$ scale clouds with temperatures of $Tgsim 2times 10^4K$ and densities of $ngsim 10^{-3}cmc$ pressure confined in hot medium with $Tsim 10^6K$ and $ngsim 10^{-5}cmc$. When the sheet is viewed face on, these cold clouds have neutral Hydrogen column densities of $NHI>10^{17.2}cms$, making them detectable as Lyman limit systems (LLSs), though they lie well outside the virial radius of any halo and even well outside the dense filaments. Their chemical composition is thus pristine, having zero metalicity, similar to several recently observed systems. Since these systems form so far from any galaxies, these results are robust to the galaxy formation physics, resulting purely from the collapse of large scale structure, radiative cooling, and sufficient spatial resolution. |