I think the only answer to this question is 'by using a large amount of imagination and suspending all credability'. The engineer said it might have been a sewer collapse, but I don't think large sewer pass under peoples houses or gardens, normally they run down the centre of the road. There must be a very large pile of earth somewhere nearby, not exactly the sort of job you could do using just a spade. That engineer must be very brave or very stupid, as he said himself "It is impossible to guarantee when the actually collapse will happen". It could have happened while he was still under there excavating.Still David should look on the bright side, he could fill the hole up with water and he will have a swimming pool.