By the 19th century, as Berlin grew and expanded to the west, Unter den Linden became the best-known and grandest street in Berlin. In 1851 the famous equestrian (horse-riding) statue of King Frederick the Great, designed by Christian Daniel Rauch, was put up on the centre strip. In the course of the building of the Nord-Süd-Tunnel for the Berlin S-Bahn in 1934/35 most of the linden trees were cut down and during the last days of World War II the remaining trees were destroyed or cut down for firewood. The present-day linden were replanted in the 1950s, they are affected by air pollution as well as by soil contamination.