Mickleham Tunnel is midway between Leatherhead and Box Hill & Westhumble. It is long and runs through the lower chalk of Norbury Park, entering the hillside immediately north of one of the three viaducts over the River Mole. Restrictions imposed by the landowner, Thomas Grissell, meant that vertical ventilation shafts could not be constructed and the tunnel portals were given lavish architectural treatment.
