It's the original rainwater reservoir from the old farm estate. The photo shows half of it. I estimate 5000 gallons capacity altogether. I was investigating an old rusted up Whale hand pump in the garden, and wondered what it was supposed to pump water from. Followed the pipe down into the bowels of the earth. It took all afternoon with a 32mm submersible pump to drop the water level that much. Now I have a ready made tank for my rainwater harvesting project, and rather bigger than I had intended.