noun
bottomless ravine; abyss
Literary expression for an extremely deep, seemingly bottomless valley.
彼はまるで万尋の谷に落ちるような絶望を感じた。
He felt despair as if falling into a bottomless abyss.
深淵 is a more common word for 'abyss', while 万尋の谷 is a classical, metaphorical expression.
From 万 (ten thousand) + 尋 (a unit of depth, about 1.8 meters) + 谷 (valley), literally 'valley of ten thousand fathoms'.