I have had a couple occasions to get into the main sewer line of the house. The first time was because the sewage wasn't flowing, something like the footfootfoot's quoted story, but without the turtles. I called the plumber after my attempts with my own drain snakes of various diameters and lengths (the biggest one is 3/8" by 50') proved ineffective. The guy brought into my basement,
a drum auger, a machine on a built-in dolly with miles of snake (well, 250') and a big, powerful motor to twist the snake and cutter head. He ran that thing down the drain and after a while, I asked him how far into the sewer the machine was. He said he was about out to the middle of the street! Wow.
When it came back out, it brought with it a large mass of what looked like black hair, but he said it was tree roots. I found this surprising, but that's just my own ignorance. Root infiltration through pipe walls is very common. Here's a picture from DB's Plumbing and Drain.
My problem was roots, which his machine removed handily, thankyouverymuch, but it sounds like footfootfoot's problem might be like the collapsed drain/soil infiltration problem. That's a bigger problem. I've read that it's possible to replace just that section of drain pipe by measuring how far the intact pipe runs, then excavating the drain only at the place where it's broken, which can be determined by measuring how far the snake was able to travel before it was stopped.
Those pictures were taken from a camera inside the drain of course. That's what I had done the second time by a drain inspection outfit hired by the city utility. A house close to mine was recently rebuilt and the owners had gas service installed in their new house. No biggie. But the installation of the gas lines meant that they needed to verify all the sewer lines in the local area to be sure that no
gas line penetrated any sewer line. Wait, what?
You can see how that might develop into a serious problem. A video inspection can yield a lot of useful information, though the inspection itself will cost some money and isn't something I can easily do myself.
I wish you luck, footfootfoot. I hope your problem can be solved without abandoning ship.