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A pond, simply put, is a hole filled with water. An ornamental pond is a hole filled with water plus an assortment of plants, some fish and a pump, which filters the water and causes it to bubble or splash in a pleasing way.

If it sounds simple, it is. Why more people don't think to build them is a mystery. Everyone who looks into the backyard of my San Francisco home and sees my water feature reacts with shock or awe or a combination thereof.

"It's beautiful!" "Is that a water lily?" "Oh look, you've got fish!" "How long did it take you to make that?"

"A day," I tell them.

"Amazing. Did it cost a lot?"

If we were discussing the latest iPod, they'd probably buy one with their next paycheck. But no matter how hysterical they get over the lily pads, none has taken the plunge, probably because they're city dwellers and don't own a shovel, an essential tool. Get one, and the rest is straightforward.

First, calculate the dimensions. Assess the size of your yard and determine how much you can dedicate to a water feature. Then decide where to put it. Factor in enough space between the edge of the pond and any landscaping features you want to leave intact. For the construction method described here, a rectangular pit lined with pliable rubber, allow a minimum of 1 foot. Then there's the question of depth. Three feet straight down is a good figure. This has to do with the feeding habits of raccoons and great blue herons. Raccoons will wade into a pond with sloping sides and devour your fish. Great blue herons, which hunt by tiptoeing around in the shallows and spearing fish with their bills, cannot tiptoe in 3 feet of water. (Although raccoons do come poking around from time to time, I haven't lost any fish to them or to herons because I've always built fairly deep ponds with straight sides.)

The Bay Area climate is ideal for water features and, not surprisingly, there are a number of businesses in and around San Francisco that can outfit you with everything you need. Acquire the following: a submersible pump with a filter basket, galvanized stakes and rubber pond liner. (To calculate the dimensions of the liner, use the following formula: Length equals pond length plus 2 times the depth, plus 2 feet. Width equals pond width plus 2 times the depth, plus 2 feet. The additional 2 feet allow a foot of overlap around the pond's edge.)

Mark the edges of your pit with stakes and string, and dig straight down 36 inches. For dense, rocky soil, a pick can be handy. For loose or sandy soil, you may need to shore up the edges of the pit with plywood as you go. You can leave the plywood in place after you have finished digging, as it provides a barrier between sharp debris and roots that might penetrate the liner. In general, though, pond liner is extremely puncture-resistant. It's also fairly easy to work with; it handles like heavy cloth.

Arrange the liner in the pit, ensuring that you have a foot of overlap on each edge to stake down. With the pit neatly lined, stake down the excess liner with galvanized skewers and turn on the garden hose. As the water rises, fold and tuck the liner tidily in the corners. (Lining the pit is like wrapping a gift box, only you're wrapping the inside of the box rather than the outside.)

If you plan to keep goldfish or koi, you will need to treat the water to remove the chlorine or chloramine. A number of products do this, such as AquaSafe, AquaPlus, etc.

The best time to buy water plants is early spring. Parrot's feather and water clover do well in pots sunk just below the surface; water lilies and water hyacinths produce colorful blooms in summer. Koi are fond of the tender roots of water hyacinth, which can lead to anemic or dead hyacinths. The solution is to stock your pond with more of them than your koi can possibly eat.

Another interesting tidbit: Many people think that koi are big goldfish. They are not. They are carp that have been bred for centuries by Japanese fish fanciers, the result being a staggeringly exotic and beautiful creature. If you look in the right place, you can find a koi that will set you back thousands of dollars. Those are fish with serious pedigrees and aristocratic lineages.

You can also pick up a perfectly good koi at a pet store for just a few bucks, and while it might not have black and white polka dots, it will still do all the things that one can reasonably expect from a pet fish.

Landscaping is as simple or as complicated as you want to make it. For a Zen look, cover the excess liner with concrete pavers. If you want to go rustic, get a truckload of small boulders and place them artistically around the edges. If you're really enterprising, bury the liner with soil, and plant Irish moss, baby's tears or blue star creeper, and when your living border grows in, your pond will look as if it formed there spontaneously. This approach requires the most upkeep, but if you like to fiddle with decorative ground cover, it could be just the thing for you.

To be honest, it may take more than a single day to build the pond of your dreams, but the digging is accomplished quickly, and the landscaping is largely fun. Once that's done, the pond simply becomes an extension of your garden, albeit more wet. It's a living decoration, a conversation piece, and a little spot where you can meditate, listening to the quiet burble of your water feature, or watching your fish swim about, munching on your long-suffering hyacinths.

So open up the phone book, and find your local pond-supply store. And if you're still holding out on the grounds that you don't own a shovel, call me. I'll lend you one.

-- A pond, even a shallow one, presents a drowning risk. If you have toddlers or very young children, or if you share your garden with someone who does, wait for the kids to grow up a bit. Enjoy the water features in the Arboretum or the Japanese Tea Garden instead. When you do decide to build, always practice common sense and do not leave young guests unsupervised in your yard.

-- Because this design doesn't involve plumbing, it is considered a landscaping feature and does not require a building permit. But choose a sensible site. To reduce the risk of electrocution, do not build your pond fewer than 6 feet from an electrical outlet. Plug your submersible pump and any other accessories into outlets protected by a GFI (ground fault interrupter).

-- Moving water will discourage mosquitoes, and fish will eat mosquito larvae, but if you have a still pond, drop a Mosquito Dunk (available at hardware stores) in the water every 30 days.

A basic pond is easy to maintain. Clean the filter every few weeks and make sure that leaves and other debris don't build up on the bottom. Do not use duckweed in the pond. It grows rapidly and requires constant skimming to keep it under control. If the water turns green and soupy, add an ultraviolet filter to your pump outflow. UV rays kill floating algae and will keep the water clear.

Equipment such as liners, pumps, filters and accessories:

The Urban Farmer Store, (415) 661-2204, www.urbanfarmerstore.com.

Sloat Garden Center, (415) 566-4415, www.sloatgardens.com.

Magic Gardens, (510) 644-2351, www.magicgardens.com.

American Soil Products, (510) 292-3000, www.americansoil.com.

San Francisco Bay Area Koi Club, www.sfbakc.org.