From the Icosahedron to E8

The regular icosahedron is connected to many exceptional objects in mathematics. Here we describe two constructions of the ${\mathrm{E}}_8$ lattice from the icosahedron. One uses a subring of the quaternions called the “icosians”, while the other uses du Val’s work on the resolution of Kleinian singularities. Together they link the golden ratio, the quaternions, the quintic equation, the 600-cell, and the Poincare homology 3-sphere. We leave it as a challenge to the reader to find the connection between these two constructions.