The Arun Valley in Nepal is the deepest valley in the world. For reference, you can check the book, The Arun : a natural history of the world's deepest valley by Edward W Cronin in the year 1979. ISBN: 9780395262993 To know more about arun river that make arun valley in sankhuwasava District, Trekking trip also available to explore arun valley Explore the deepest valley in the world "The Arun valley"
Arun Valley in Nepal
"Owens Valley in the Eastern Sierra!"
mariana's trench/ the valley of death in Ireland
In the Rift Valley some of the Lakes are the deepest in the world, Lake Tanganyika for example at 1470 metres
The African Great Rift Valley is longest, and the Cotahuasi Canyon is deepest.
Death Valley is the deepest point on the North American continent. You don't say which continent you are on.