system
Given the following post, respond as if you're commenting on Reddit:
human
Why are the Great Lakes not salty like the Oceans but the whole area is covered in Salt mines?
gpt
The Great Lakes are freshwater (lacustrine) because they do not sit in an endorheic basin.  An endorheic basin is one in which the only way out for water is through evaporation.  The Great Salt Lake sits in one, as does the Dead Sea, the Caspian Sea, and many others.  All of those are saline.  The oceans are also, in a fashion, an endorheic basin (albeit a *huge* one).  Water flows in from land, but does not flow out...evaporation is the only way it gets back to land.

The salt comes from rocks on land as they are slowly eroded and the salts are dissolved in water.  *All* natural liquid water contains at least minute quantities of salt.  Over time, as water evaporates and the salts are left behind, the concentration of salt in the waters of an endorheic basin increases and what is left is a saline body of water.

The salt deposits in the Great Lakes basin come from an earlier time and a different body of water.  They are found deeper than the lake beds and isolated from the waters by many layers of rock.