One of our reserves is fed by underground spring water. The water is of such good quality that we have fresh water sponges living in the ditches.
They come in many shapes and sizes and surprisingly they are actually animals. These sponges are a mass of cells within a porous protein skeleton which contains silica and calcium. They feed on tiny organisms by filtering them from the surrounding water.
You can find out more about sponges here; http://www.bbc.co.uk/dna/h2g2/A839144