I have recently installed drainage in my garden and I am on the final preparation stages, before laying the turf, however, on compacting down the soil I have come across an area, about 3ft square, that is like a sponge. Every time I walk on it I can see the ground around my feet 'wobble', and if I try to press it down, the ground just raises next to my foot! I am at a loss as to how to get rid of it (or what causes it). Can anyone help?!!!