The Surrey Hills boasts a rich diversity of natural and man made features associated with water, including springs, ponds, canals and wooded ghylls. Most are quite small, although larger ponds such as Frensham and Silent Pool are prominent features and visitor attractions. Ponds have featured in the landscape for centuries, often created to satisfy the needs of industry (mill ponds and hammer ponds such as Thursley), agriculture (field ponds), sport (duck decoys), and to act as a focus for communities (village ponds). There are also lots of ponds that can be seen on heathlands and commons derived from bomb craters and from mineral extraction.