What would you do with a Mystery Tank?

We've inherited a big, old, metal tank. It's a perfect cylinder (minus a dent or two), 7 feet long, 4 feet in diameter. It's covered in surface rust (inside as well, judging from the water we pumped out of it). It has 3 holes on the top side, and a drain hole at one end. 2 of the top holes are open, one is plugged (the bigger one) and the plug seems seized with rust. It can hold ~ 660 gallons.

My suspicion is that some previous owner tried to set up a diy heat pump or solar hot water system. The tank was in an unheated space under a balcony, with hot air ducting coming down on two sides. I was also buried under 3/4 inch drain rock. Truly though, I have no idea what it was for. It was almost full of water when I inspected it. The water seemed clean, with some rust. I sniffed it, drew some out to observe the colour, and poured a little on a coffee filter and let it evaporate, there was no residue. I have not tested the water.

I pumped the water off property to a roadside ditch that runs downhill from our land. The next couple of properties down slope don't grow any food, and the water seemed clean. Removal by a septic service would have cost $300. That all being said, I have no real knowledge of the history of the tank, where it came from, what was stored in it, how it has been used.

I can always try and have it hauled off for scrap, but I'm wondering if there might be a better use for it around our permaculture homestead to be. What would you do with it? How would you make sure it was safe to use?

What types of tools do you have, do you have a welder and a cutting torch or grinding wheel? Is there any smell of fuels? Can you see any name plates, stamps or tags on it?

I have none of those tools, but I can borrow all of them. There is no oil smell of any kind, merely a faint smell of rusty water. There are no identifying marks of any kind, that I've been able to find.

We have one. It was a propane tank for heat. We own it, and havent used propane for 5 years. We have an indoor wood burning heater. My Hubbie is going to use his torch, and cut it in half, and make a large outdoor wood burning heater, that will also heat our water.