I have a septic tank in my yard that is no longer used. Can I leave it there? Should I remove it? Fill it with something? I'm not entirely sure of the building material used to construct it; I assume whatever was the customary materials in the pacific NW in the mid to late 50's.
You'll have to check codes, but I believe the usual procedure is to fill them with something like sand - since it's a septic tank, and since (I presume) you had it emptied before leaving it to rot, you probably don't need to clean it, just get it filled.
You can NOT just leave it there - eventually it will rot away and collapse, presenting a hazard at some unknown future time.
You might call around to the local septic companies and ask them what the usual procedure is in your area, they should know who can handle the fill-in of the tank properly.
Standard procedure in this area is to crush steel, wood or plastic ones and fill the hole, and to fill concrete ones with stone. In all cases pumping the waste first.
A common reason that has not been mentioned yet is to prevent people from entering the old tank - there are NUMEROUS tales of using the old tank as a place to put grass clippings, kid drops the rake in the hole, goes in after it, asphyxiates, etc...the top search result is this one where the kid lives and 3 rescue workers died, while a 4th was injured.