the path petered out very quickly , leaving art standing on a raised hummock of broken down and rotting reeds , beneath which the ground felt dangerously soft and spongy , in between two cress beds which , at this time of year , without any sign of their future crop , appeared as two unfathomable empty pools , their parameters meticulously marked by the aid of a thin , green plastic twine , which was stretched , with the aid of supporting poles , all around the boundaries of the water , at a height of just a few inches above ground level .