What to do with a hole in the ground next to septic?

Any suggestions from a permaculture design or prepping perspective for what to do with a hole in the ground near a septic system? It’s 22 feet x 15 feet, and between 4.5-6 feet deep (it’s on a gradual hill slope). It’s in the middle of a small yard out front of the house; I got rid of an underground propane tank and now have a nice free hole there.

But with it so close to septic, I’m out of ideas. The septic tank is 3-5 feet away from one of the hole’s short sides, and the leach field is about 10-15 feet away from one of the long sides. We were thinking about a pond or underground cistern for rainwater harvesting, but we wouldn’t trust the water to be safe. We thought about a root cellar or half-underground greenhouse (walipini), but same concern about toxins coming in. But maybe toxins are not really a concern for a root cellar or greenhouse as long as we support the walls enough?

Welcome ideas!