Bleach and hot water. That's it.

Yes. Mix a tablespoon of bleach per quart of water, scrub the inside, let it sit for 15 minutes, then rinse like crazy. Make sure it dries completely before storing next time, or this will happen again.

Mold in a cooler is basically a biology experiment you didn’t sign up for. The bleach kills the spores, and hot water helps lift the gunk from the plastic. Don’t bother with vinegar or baking soda—they’re fine for surface smells but won’t kill the deep mold that’s taken root in those corners and drain plugs.

Important: after bleaching, rinse with plain water at least twice. You don’t want bleach residue near your food or drinks. Then prop the lid open and let it dry in the sun for a full day. Sunlight is a natural mold killer, plus it gets rid of the smell.

If the mold is really stubborn (black spots that won’t scrub off), try a paste of bleach and baking soda. Scrub with a soft brush—no steel wool, you’ll scratch the plastic and give mold new hiding spots.

The real fix is storage. Store it with the lid cracked open and a paper towel inside to absorb moisture. Future you will thank present you.