Earthly Delights part one

This afternoon I have been pottering around in the garden: sweeping the garden path, setting a bench underneath an archway, churning the hazardously evergrowing compost heap, rigging a six foot scarecrow (to make it realistic - the birds round here are smart), and I stumbled upon a fascinating thing.
By the garden fence are some concrete foundations. They've never been touched and my Mother reckons they're from an old shed and has covered them up with leaf mould. After sweeping the leaves and overflowing compost heap away I found that the concrete is a couple of square meters in area and in the centre is a little square tile made from a different type of stone altogether. It is a matt white with a speckling of green and I have never seen anything ike it. On closer inspection I noticed that it had a carved handle to one side, just a small raised section, enough to get your fingertips under. Now what could this be, I thought. What a strange thing. I know there is a bomb shelter over the back so maybe this was an additional entrance. A considerate old boy must have had it built to protect his family in case the Nazis started bombing green belt areas of London, but I presume that was before the fence was put up and this shed and foundations were established. Alternatively, the little area could have been for a psychotic suburbanite to bury their victims. I thought this was exciting.
So I levered the little speckly tile up. This is a lot easier said than done as I had to scrape away the moist and claylike soil with my fingers and brush away the earth before inserting a pair of sheres into the gap and pulling. There was a dull hollow crunch as the old material gave way and a cloud of dust emerged as the surrounding mud caved in.