
They are working with heavy machinery.
Indeed, they’re building a TUNNEL.
Money Heist Season 1, Episode 11

And in all those centuries,
we have never sealed the TUNNEL.
Game of Thrones Season 4, Episode 7

Cold, gloomy, and easily accessed by
a Frenchman through a TUNNEL.
The Big Bang Theory Season 9, Episode 8

Lord Snow here recommends
sealing the TUNNEL,
Game of Thrones Season 4, Episode 7

Who made that TUNNEL?
Money Heist Season 1, Episode 8