The phrase "deep basin" refers to a large, low-lying area that is characterized by its substantial depth. It is like a big bowl or hole in the ground that is significantly deeper compared to its surroundings. Full definition
Unlike other deep basin tubs that you either have to hang up or put away in a closet, this one will fit neatly in any small space. (earlymomentsmatter.org)
A period of diffuse extension created the rift valley in the late Cretaceous, roughly 100 million years ago, Lloyd said, and more focused extension then created deep basins like the Bentley Subglacial Basin and the Terror Rift in the Ross Sea. (sciencedaily.com)