If it is full of rubble it is far more likely to be a sewage leach pit (or runoff from gutter) than a fresh water cistern.

Thanks, (once again), Brickeye! I should have been a little more specific - I'm assuming it was for gutter run-off collection, also. The clay pipe is still there that delivered the water, it angles toward the corner where house & porch meet about 4' from end of pipe. Only the end is visible, so that's a guess for now. Also is a small (1-2") hole in outside facing away from house, slightly lower than the other - I'm thinking a pipe went there to direct overflow away from foundation?
The whole thing looks to be no more than 1' from house foundation - next to the root-cellar.
From what I've read, its unlikely it was potable water, even then, right? For cleaning & watering only?
Sorry to be such a pest! Mysteries drive me nuts...
:-)

How about start by putting a ladder down there and seeing if its going to support your weight?

I'd see if you can get it cleaned out. Cisterns are great for watering the garden and lawn. There are still some houses in my area further from town that have cisterns and have water shipped in for the household needs.

Thanks, clg! The ladder is a good idea - & gave me a head-thumping moment. Better yet, I'll start with a level-rod! That way I can "prod" & measure depth @ the same time! (Raised & trained by Land Surveyor Dad, worked in surveying & construction staking most of my life). Still have equipment, since he passed & family biz passed along-side.
Further research gives me the impression that I need to go to the (GASP!!) root cellar/crawl space to look for signs of a pipe (w/valve, initially) coming in down there. Scary place, unless you think spiders & cobwebs are groovy. I do NOT. But the desire to search will outweigh the, umm, "apprehension". Lol!
Thanks!
It occurred to me, also, that our (Dad trained DH, he's RLS, now) field of work could make our advice & opinions useful to someone - property line questions, etc. I'll post that offer soon, so many have been helpful to me!
Thanks, again!

If you manage to clean it out you will likely need a submersible pump to move the water (unless you want to go the bucket and pulley method).

Or put in a hand pump. I have one in my kitchen running to my spring cachement basin to use when the electric is out. They'll pull to 25 foot depth. Some even come with threaded nozzles to use with hoses and have mechanisms to smooth out the water flow between pumps.

We've been puzzled by a large ceramic pipe that extends out from the dirt under our house near the center of our cellar. It appears that someone filled the pipe with concrete, but just beneath the pipe a channel seems to have opened and water pours under the pipe and into our cellar floor when it rains. Judging by the foot or so of the pipe we can see, it seems to run toward the part of our foundation where there's a nearby downspout. Could this be part of a cistern? If so, would the cistern be somewhere around where the pipe ends? It's dirt there now (or I should say, mud). We need to either figure out a way to use the incoming water or stop it from coming in, as it's keeping our cellar area wet--not a good thing. Advice anyone?

It's hard to tell, Debbie. It might be the remnants of an old soil pipe for household sewerage. Most water catchment cisterns are buried on the exterior of the foundation, and I can see no reason to put it inside, unless it is located in a part of the cellar under an add-on. If you have an old farmhouse, it also could have been where spring water was piped into a trough for diary cooling purposes but I doubt that is the case if it is large. My spring pipes are old cast iron ones about two inches in diameter.