Installing underground water tanks residentially

Is it possible to buy huge underwater tanks and have these installed on your land? If you have a large garden for example, it could be plausible to dig it up, install a tank the size of a cargo container, put the plumbing in and re-bury it. Maintenance may be an issue down the line, but otherthan that you've got a fantastic source of multi-purpose water (although I wouldn't drink it!)

Does anyone know if equipment like this is sold for the residential market? And would you need some kind of planning permission?

I think it all comes down to land. If you have acres of land, and plan to use a lot of water gardening, then I absolutely think this is a good course of action. If you have a large private estate, I'm not actually sure planning permission would be necessary, and if it is I don't think it would be too difficult to achieve.