The US television drama Lost first aired in the United States on September 22, 2004 with the most expensive pilot in TV history. The series went on to attract US audiences of just under 24 million viewers and, before long, Lost had become an unprecedented hit, winning Emmys and keeping fans enthralled with a litany of tantalising plot twists and mysteries.

Over time, however, some viewers tired of the expanding web of mythology and the lack of concrete answers.

Despite fans' frustrations, Lost formed a cult fan base to rival that of The X-Files and, if you had the patience, it became one of the most gripping high-concept US TV shows of its time.

Fifteen years on, here are some of the key mysteries introduced over Lost's six seasons and how - at least as far as we can work out - they were resolved:

WARNING: CONTAINS SPOILERS FOR ALL SIX SEASONS

1. The Smoke Monster