"Discovering The Hatch" is an orchestral piece on the Via Domus soundtrack. It plays during Elliot's discovery of the hatch in "Via Domus". The mysterious percussion piece serves as John Locke's Via Domus theme.
Having followed markers through the jungle per Locke's instructions, Elliot meets the man himself at the still closed Hatch, which sports a strong yellow beam of light coming out of its main window. Locke marks that Elliot has "found another one of his secrets" (the first being Elliot's knowledge of Locke's former paralysis) before the two talk on fate and destiny, as Locke normally does.
The piece is directly inspired by Through the Locke-ing Glass, which serves as Locke's mystery theme in the show proper.