Are there other holes around to inspect? What critters have you seen? The link below is from Clemson, but I'm sure most of the critters are the same in both areas.

Here is a link that might be useful: identifying who dug

I don't see any other holes. The only way I found this tunnel was digging down 12" to plant a pepper plant. After I got down about a foot, the soil felt "hollow" and just started to crumble away revealing the huge tunnel. I have since filled it up with sand, but just finding it gave me shivers!

could it be an armadillo? i have one here and he has made a tunnel along side of my house that i tried to fill with water and it just went on forever.this is the most awful pest ever they did up things over and over and my yard is full of holes

So, you were digging and discovered this chamber in the ground? There wasn't a hole already present? Did you make that mound of soil with your own digging? (Just trying to clarify this in my mind.)

Animals will have an entry hole to their nest and maybe even another secondary hole or more.

Armadillo nests are usually found among the roots of a tree or large shrub, or under a slab of some sort...they seem to like something over their heads. They will, though, excavate a yard like crazy in search of grubs, worms, etc. That's not to say that they, or any animal, might not change the rules to adapt into the environment.

Rats won't build a nest out in the open, even if it is underground. I doubt that that's the digger here.

When I lived along the coast of SC, we had pocket gophers. They came to mind with your situation. Perhaps an option to think about?

If you didn't see any signs of the tunnel before your own digging, nor any obvious hints of recent animal digging, then I'd say that this location is no longer in use. Keep an eye on the area, though.

In recent years, were there any trees in this location?

Here is a link that might be useful: Just for information

So, you were digging and discovered this chamber in the ground? There wasn't a hole already present? Did you make that mound of soil with your own digging? (Just trying to clarify this in my mind.)

Correct, there was no hole before, just a regular flat part of my backyard and I did indeed make that mound with my digging.

If you didn't see any signs of the tunnel before your own digging, nor any obvious hints of recent animal digging, then I'd say that this location is no longer in use. Keep an eye on the area, though.

Will do. I actually did plant some peppers in the hole, so I'll definitely keep an eye out for the area. No trees here recently (last two years at least).

Gophers sound a lot better than rats :-)...but the tunnel was mighty wide - about 8" and went several feet down into the earth. I was just waiting for an anaconda or something to pop out and get me ;-)

I have seen many similar holes in my yard, but so far no plants have been harmed. I feared it might be voles, but lots of harmless creatures live in holes too, and so far we are all getting along together...fingers crossed!