The seminal work of Howard Garland in the 60's, proving the conjecture of Serre on vanishing cohomology of lattices in $p$-adic simple Lie groups reveals that high dimensional expanders (HDX) have a ``local to global" phenomenon which does not exist in graphs. Furthermore, the work of L. Lafforgue, generalizing Drinfeld's work from $GL(2)$ to $GL(n)$, called for the construction of Ramanujan complexes. These high dimensional simplicial complexes led to the development of a rich theory with applications in CS and math.