A Sinking Suspicion

I found out that there is a button on my laptop that controls the wireless antenna. Somehow it got turned off and I haven't been able to use the internet to post this past week. I didn't know what was going on (nothing new there), but luckily Jane figured it out. More about my incompetence, and Jane's superiority later, here's the real story.

So, I'm was blithely about my business at work last week when I get this call from Judy around one in the afternoon. She indicated that she had an important FYI to tell me. I should preface this story with the reason Judy and Elroy were home.....they had finished with their second-quarter exams and had a couple of days at home. Exam week is a hectic time and Jane and I spend a lot of time and effort coordinating their odd-houred exam schedules and after-school activities. The typical who picks who up and who goes where when, so I wasn't really surprized when Judy called. I expected something like.....I need to be at school can you drive me or what are we having for dinner tonight.....can I make one of the bundt cakes you got for christmas? I was wrong!!!

So Judy started her story. She had been taking Astro for a walk and was with him in the back yard when she noticed a depression in the ground. Being a curious girl she wondered why there was a depression in the ground....and as a natural progression of that thought......what would happen if she jumped up and down on this area. Now I have to say, and maybe I'm just more naturally cautious, but I would have use a stick to probe the area or more likely had Astro walk over the area first. Anyway, the earth gave way under her weight and she plunged into a gaping watery sinkhole that swallowed her up to the waist.....or the ankle, I wasn't quite sure. But she continued saying that she and Elroy had measured the depth of the sinkhole at around 4 feet. A four foot deep sinkhole in my backyard??? In my mind I'm seeing the both of them standing on the weakened edges on this sinkhole, jabbing a stick into just-about-ready-to-cave-in edges of the sinkhole followed shortyly by them hugging onto each other as the earth slowly gives way under them and they plunge out-of-sight. Needless to say, I told them to stay away from the sinkhole until I could come home to look at it.

After some examining it seems we have an old, very old, septic tank that is now crumbling and causing a growing sinkhole. So my quesion is what should I fill the sinkhole with?