Lead

This material is rarely used in practical ponds today, and water from a lead-lined container should not be drunk. It is , however, supremely long-lasting, as demonstrated  by the Roman baths at Bath, England which were made with a 19mm layer of lead, dressed with stone - there is no record of this ever leaking. Lead is now available in rolls for roof work and can be used, when appropriate, to cover vulnerable or unattractive materials at the water's edge. After a few years it takes on a stable, mellow and attractive finish.