There were 2 rectangle sprites I noticed. The brown one is Dig Dug's shovel and the red one is his pump. If you look closely at the Dig Dug sprite when he is digging or pumping, you'll notice a little cutout in it. On a regular NES, the rectangles would be drawn behind the Dig Dug sprite and the color would only be seen through the cutout, if that makes sense. They use that technique in other games as well, though I'm not sure why.

With things like those, I set the depth of one to be a little higher or lower or even a different layer depending on which one I want showing, otherwise, if you set the faces to the same depth, you end up with Z fighting.

I forget exactly when, but it was while digging, as opposed to just walking through tunnels already dug, since that is when he pulls out the shovel. It might have been just one frame where I was digging a specific direction like up or down.