The Lower Mines are the locations in Pajama Sam in "No Need To Hide When It's Dark Outside".
The lower mines is accessed by going into the tunnel that says "Gold". While it is accessible in all versions of the game, it is only important to go here in the Secret Mines storyline.
In one stopping point in the lower mines is where the gold is. Sam will need to use a pickaxe to excavate some gold out for himself (and King after Sam gets two pieces). Sam is only required here to go here in the Secret Mines storyline.