Season 2 · Episode 4

The Survey Corps goes back to Drithane to measure the crossing itself. The engineer sent to do it packs the one instrument Eden never had a form for. Everyone on that world watches the drithle. Nobody has ever once listened to it.

• S02E04C01 The Undersong of the DrithleDrithane (Bríd System), Gleann na gCaorach valley chainThe Survey Corps returns to instrument the crossing itself, and a Chthonari engineer's kit includes the one instrument Eden never had a form for — a vessik, deaf to air and fluent in stone, who cannot watch the drithle and is the only resident of the valley who can hear it.