From Lostpedia
Locke looked into its eyes.
Mr. Eko stared into its heart.
The "Monster", aka security system is one of the many unsolved mysteries of Lost.
Damon and
Carlton have promised more of it next season, and hinted that we may have seen it in
season two but didn't realize what it was. Whatever it is, this tree rustling, ground shaking thing is still an enigma to the survivors.
