It's the cavity's ECHO,
the subsurface humidity.
Money Heist Season 3, Episode 8
and ECHO with the cries
of the wounded."
The Big Bang Theory Season 5, Episode 15
(GUNSHOTS ECHO)
Westworld Season 3, Episode 8
( gunshots ECHO )
Westworld Season 2, Episode 10
(SHOTS ECHO)
Westworld Season 3, Episode 8