Complex sentence: Through Thatcher's written memoirs, Thompson learns about Kane's childhood, which began in poverty in Colorado (his parents ran a boarding house), until "the world's third largest gold mine" was discovered on the seemingly worthless property his mother had acquired.
Split into simpler sentences: Through Thatcher's written memoirs, Thompson learns about Kane's childhood. These flashbacks reveal that Kane's childhood was spent in poverty in Colorado (his parents ran a boarding house), until "the world's third largest gold mine" was discovered on the seemingly worthless property his mother had acquired.