All septic tanks have to be emptied and that includes all chambers, every year, (though a lot of tanks go 30 years before the owners empty them!). They must, therefore, have an access to all chambers. Sometimes it means that the entire concrete slab top has to be slid aside and I would guess that when this was done, someone on the farm who had the job of getting rid of an old tyre, threw it in the tank before the lid was slid back.
Septic tank gases include Hydrogen Sulphide which turns into Sulphuric Acid on contact with a damp surface and air. This gradually eats its way into the fabric of the concrete, destroying the cement until the lids have no stregnth left. Every year, many people die when septic tank lids collapse - even under the weight of a person - so it is vital that the tank is checked and, if not used anymore, emptied and filled in before a worse accident happens in the future.
It can be serious mate. I had a survey once to find the location of the cesspool. I found it when I pulled onto a disused part of the stackyard and lost a front wheel into the tank!. I made sure that the farmer pulled me out very carefully!
