system
Given the following post, respond as if you're commenting on Reddit:
human
Why are there so many salt deposits in the Great Lakes? Wouldn't this be counterintuitive since they are freshwater lakes?

Furthermore, is it more common for salt deposits to be under land or underwater?
gpt
The salt deposits are remnants of ancient seas. Much of the great lakes region is underlain by sedimentary rock deposits that originated from these seas as well. 

A common linkage between the salt deposits and lakes may be due to topography: lakes form in low basins on the landscape. Seas form similarly. Since the catchment basins that feed water to these areas are now above sea level they fill only with fresh water.