DIAGNOSIS That's baked-on mineral scale and carbon from direct heat, especially if you've used a high flame or electric coil burner. The aluminum base discolors easily, but it's just a surface stain and doesn't affect safety or performance.
FIX Don't use steel wool or harsh abrasives—you'll scratch the aluminum. Try this gentle method: 1) Make a paste with baking soda and a little water. 2) Spread it thickly over the stained area and let it sit for an hour. 3) Use a non-scratch scrubby (like a blue Scotch-Brite pad) and warm water to gently work the paste in circles. 4) Rinse. For tougher stains, you can use a diluted vinegar solution (1 part vinegar to 2 parts water) and let it soak for 30 minutes before scrubbing with the baking soda paste.
IF THAT DIDN'T WORK For persistent, dark stains, Bar Keepers Friend (the liquid or powder version) is safe for aluminum and very effective. Follow the directions on the container, rinse thoroughly, and dry the pot completely afterwards.
HEADS UP To prevent this, always ensure there's at least 2-3 inches of water in the bottom during use, and avoid cranking your burner to its absolute maximum. The stain is cosmetic, but a clean pot lets you monitor for any real scorching or debris that could interfere with heat transfer.