I am friends with a retired sewer worker here in Portland. When he learned I used a metal detector, he started telling me some of the things he had found in sewer lines while they were being "cleaned". Periodically, the lines clog up with debris and rocks, and a high-powered vacuum truck is used to suction out debris from traps. The primo job was standing in the back of the dump truck with a shovel and making sure the load of debris was distributed evenly. He told of finding as many as 10 diamond rings a day, up to 25 wedding bands a day, plus assorted coinage, mostly silver, that would be snatched from the pile before being covered. He's been retired for several years now, and he refuses to let me see any of the loot. Even though he loves making outrageous stories up, I sense he's not kidding about this.
Here's a tip for detector users anywhere: where does your city dispose of its overflow sewage waste? Find out where those dump trucks unload the loads, and you might just find some of the stuff that got covered up before someone could pick it up. If it's into a stream or river, just downstream from that outlet during low water times (like now, for instance) can be, uh, magical. The needles can be a pain, so prepare with protective-soled footwear. This is NOT for the faint of heart or safety conscious.