Seepage Pit - Sink Hole

A funny thing happened on Wednesday morning in the rain.  Nancy thought she heard something fall in the morning in the front yard.  When she looked out she saw nothing.  Later when she went out to get the mail she found a hole in our front yard.  Not just any hole but a deep hole without a bottm, or so it seemed in the rain.  Next to the hole is one of the seepage pits for our septic system.   What is really odd is that  we can't see when the soil went.  It was any awfully lot of dirt that just disappeared.  Below are a few shots of the inside of the hole.  You can see in the pictures that there is a large gap between the seepage pit and the surrounding soil.  I'm told that the space around the pit is supposed to be filled with gravel.  What is oddest to me is how the thing has been covered with a a big void space inside for almost 15 years.  

This is looking down into the hole you can clearly see the pit and the gap around it.  The hole goes down about 16 - 18 feet.

This shot shows the gap pretty clearly.  As near as I can tell there is a space about 9 to 12" wide all around the seepage pit.  I suppose if we don't get it filled soon it will probably start to smell.