Complex sentence: One of the world's most significant and richest sources of caesium is the Tanco Mine at Bernic Lake in Manitoba, Canada, estimated to contain 350,000 metric tons of pollucite ore, representing more than two-thirds of the world's reserve base.
Split into simpler sentences: One of the world's most significant and richest sources of the metal is the Tanco Mine at Bernic Lake in Manitoba, Canada. The deposits there are estimated to contain 350,000 metric tons of pollucite ore, which represent more than two-thirds of the world's reserve base.