For 50 years (1170-1220), pilgrims wishing to see the shrine of Thomas Becket were directed to the crypt, for his remains were buried here. From 1220 until the destruction of the shrine in 1538, pilgrims visited the new shrine in the Trinity chapel. The main steps approaching the choir became known as pilgrims' steps. The stone became very worn from the shoes, feet and knees of many thousands of pilgrims who took this route.
What to see:
undulations and disfigurement in the stone forming the steps in the north and south aisles (Images 1 and 2)
similar undulations appear in floor of the Trinity chapel (Image 3)
Sources: standard cathedral sources