[gleeaikin]

I wonder if there is something similar under such rivers as Mississippi, Nile, Ganges, etc?

[Eric in the Ozarks]

There is evidence of an underground river under Lake Superior. Several of the streams from the Boundary Waters partially disappear before entering the big lake.

[blam]

The water volume of the Amazon River at the discharge point, is ten times greater than the next ten largest rivers in the world.

[hinckley buzzard]

I would bet on it for the Mississippi. Old Man River is obviously the benefactor of an ancient fold in the continental plate, so that would go deep and drain underground aquifers as well as surface runoff.