Water can move dirt without you knowing it, This is a picture of a sewer repair I did in Portland Oregon. The sewer was installed in 1930s, and over the course of 75 years a small amount of water made a cave around and under the pipe, so large we had to truck dirt in to fill the cave. The sewer still worked without a problem and the home owners didn't know
that a cave fifteen foot long, three feet tall and three feet wide was there.
  In the course of replacing some concrete sidewalk, the concrete crew found a hole in the ground and talked the owner into getting a sewer scope done. The concrete pipe had leaky joints and some cracked joints, but the sewer till worked.
 The picture of the sewer is at 12 feet deep, and its the sewer lateral from the sewer main it ran fairly flat, the sewer line form the house ran with a vary steep slop to the sewer lateral, Ten feet or more of slop and the soil had a large amount of sand in it, but did not drain water vary well, just the right amount for this to happen.
 In most repairs there is some ground movement because of water leaking from the old lines, in this case the water had moved the dirt out into a 30 inch concrete sewer main, We think that's what happen to the dirt!
 Like I said in most cases the leaking of sewer lines just turns the dirt blackish purple, and the dirt looses its strength for support of the sewer line. That's when bellies in sewer lines happen. In rocky ground or cobble the water moves through the ground well, but if the sewer line has a hole in it or large cracks, the course sand and small rock will run down the sewer line and dam it up.
 This cave is unusual but not uncommon. Will post more as I find them. Thanks! are web site is   oregonsewerservice.com