The liner at the back of the hole is part of an exisiting bog garden.

Obviously, digging the hole is the hardest bit of making a sunken pond.

The soil that you dig into will probably be compacted, but any soil put back in the hole to correct over-enthusiastic digging will be loose and will settle under the weight of water. So be careful not to dig out too wide a hole, so you can maintain compacted and firm support for the finished pond edge.

Scrape out a shallow ledge around the hole's edge to accommodate the depth of plywood strip. The aim is for the top edge of the pond liner to be slightly proud of the surrounding ground (see the picture of the finished pond). Establish the top height using a length of wood and a spirit level (see picture - right) as you dig.

The pond should have a deep section, at least 45cm (18in) deep (preferably 60cms (2ft) deep in colder areas), so that even if the water freezes over on top, it should remain liquid in the depths, which will help amphibians survive a bad winter.

The pond's sides should be carefully shaped to avoid a saucer-like cross-section. The problem with a saucer shape is that much of the water is very shallow. Steeper sides result in a greater volume of water in relation to the pond's surface area, which reduces the likelihood of temperature fluctuations. Pond life thrives in a stable environment.

However, vertical sides to a pond are difficult to create because the soil tends to collapse as you dig, so aim for as steep a sloping side as possible in this case, incorporating a planting shelf part-way down for marginal plants.

Make one portion of the edge more gently sloping than the rest so that creatures can enter and leave the pond easily. After all, it's quite common for hedgehogs to fall into ponds and drown because they can't scramble back out; frogs and toads find steep sides a struggle too.

When you've dug your hole, remove any tree roots and stones from the surface of the hole.

What could go wrong:

• You could hurt yourself with enthusiastic digging. Really! Pace yourself.
• Loading too much soil onto the spade. The resulting backache isn't worth the perceived time saved. Use a border spade which has a smaller, thinner and sharper blade and is lighter and easier to use.
• You could reach the water table so the hole fills with water, which is annoying and messy. Bail out the water with a bucket, or wait for a dry period to dig the hole.