Complex sentence: El Teniente mine is undergoing since 2011 an structural project called New Mine Level (NML) consisting on expanding the mine deeper into the hill at 1,880 meters above sea level.
Split into simpler sentences: El Teniente mine is undergoing, since 2011, a structural project called New Mine Level (NML). It consists of expanding the mine deeper into the hill at 1,880 meters above sea level.