The phrase "vast reservoir" refers to a large amount or quantity of something. It suggests that there is a huge supply or collection of a particular resource or substance. Full definition
Children with vast reservoirs of background experience share space with peers whose world is circumscribed by the few blocks of their neighborhood. (ascd.org)
The now - solid Tava deposits apparently flowed from vast reservoirs of once - waterlogged sand, some of them containing more than 1 million cubic meters of material. (sciencemag.org)
Most of the water in the universe floats in vast reservoirs called molecular clouds. (webbtelescope.org)