Minecraft wiki entry for Development_versions, associated text: Terracotta and the dyed forms of terracotta no longer generate more than 15 blocks deep, if the mesa is more than 15 blocks above sea level.
In addition to the normal 2 blobs of gold ore below Y=32, attempts to generate 20 blobs between Y=32 and Y=79.
Generate the new type of mineshafts, described below.