Okay, after years of toiling (maybe a bit of an exaggeration), I finally located the hatch to the septic tank. Though it was a bit of a chore, it makes sense that once you locate the tank it's just a matter of time before you find the door. You may end up tearing up your backyard, but at least there's a light, albeit a small one, at the end of the tunnel. I foolishly thought I could rely on my memory to find said hatch and tried to locate it by landmarks, but once again, my memory failed me.
I actually had two competing piece of information running through my head: on the one hand, I recall the previous owners telling me it was 16 feet back from the southeast corner of the house, while on the other, I thought it was lined up with our mountain ash tree. I was wrong about the tree. I started digging based the tree and managed to locate the near edge of the tank, so I knew at that point I had to move in a southerly direction. I then employed the 16 foot piece of information and located the tank, and from there I just started to widen the hole. The problem was that you can widen the hole in many directions, and I had to take guesses as to which one to take. I went with about a foot in each direction and before long I found it, but it was a pain.
One thing that really made it frustrating (beyond frustrating, actually) were the rocks. The tank is about 2 feet down, but add in plenty of rocks, and big ones to boot, and I was starting to lose my mind. For all it's worth, once the initial hole is dug, expanding it was much easier, but it's still a pain. I decided to take the advice of the septic guy and install concrete blocks to raise the level of the opening, but that could be another story in and of itself. For now, I'm just glad I found the hatch. I have a couple of weeks before they come to pump it, so I can ignore it for now and direct my attention to other pressing matters.
Until the next time, thanks for reading.