My hole is about 8 inches wide at top and goes straight down the side of my retaining wall about 6 feet. I cannot tell if it tunnels out at the bottom but probably does. It narrows to about 4 inches diameter at bottom. Same hole dug last year but in the fall right beside it. I clogged it with bricks. Now the critter got me again. It doesn't bother the garden which is in raised beds right in front of holes.

It would be easier to figure out how to get rid of it if you knew what it is

A six foot deep hole only eight inches across? If your wall is near water, possibly you have a nutria burrow, although they are normally active year round.

Here's some information on beaverrats

http://www.ces.ncsu.edu/nreos/wild/pdf/wildlife/NUTRIA.PDF