Dragging the lake?

What's the meaning of "dragging the lake"? I hear it all the time, but I've never understood what exactly it meant. Also, I looked up "catch 22" but the definition left me even more confused... Can anyone help me, please?

One definition of "drag" from the American Heritage Dictionary is "To search or dredge the bottom of a body of water." People do this when trying to find a corpse, a gun, a sunken boat, or something else that might be lying on the bottom and not visible through murky water.
A catch-22 is a paradoxical rule that has the same effect as "Heads I win, tails you lose." Whatever happens, you lose either way. Joseph Heller's book "Catch-22" introduced the term.