concrete

You can build a pond using a flexible liner or you can concrete the base. You mix the concrete using cement, gravel, sand and water and throw in a little waterproofing agent too (as best as I can remember the procedure).

Having prepared a flat base and removed any loose material you tamp down a layer of around four inches of cement and then smooth off the surface as if plastering.

Leave it to 'go off' – which just means 'to dry', not to wander away. Then, just add water. Voila, your pond.