Not only mermaids can go under the sea...

What was once a preserve of marine scientists or James Bond films has become a popular tourist activity.

About 16 million people have enjoyed a view of the exotic underwater world from a submarine since underwater tours began as a viable tourist business in the late 1980s.

Tourist submarines - whether based in Hawaii, the Canary Islands or the Caribbean - can take holidaymakers to depths of between 20 and 40 metres.

They are often painted white or yellow and invariably dive down to the Beatles' tune: "We all live in a yellow submarine."

"Cruise ship passengers, especially in the Caribbean, are our most important clients," says Dennis Hurd, president of Atlantis Submarines based in Vancouver.

About half of the 30 submarines currently operating worldwide come from the Canadian city. Most of the other boats are built in the Finnish city of Turku.

The constructors of these special tourist submarines with their large panoramic windows may live in cool northern countries, but the vessels operate mainly where the sun shines all year round and where there is a coastline with beautiful fauna and flora.

American visitors to Hawaii are especially keen on the subs. There are days when up to five submarines per day take 2 500 visitors underwater. Until 1994, the submarines could at the most take 48 passengers. Then bigger vessels were built, with one of them based in Waikiki Beach in Honolulu taking 64 passengers.

Security standards are high. Submarine crews are constantly in touch with a guide ship by radio. Entry and exit holes are bigger than on military submarines and every seat is next to a window.

The air pressure regulating system on board means that passengers have no feelings of dizziness or ear problems.

The first such submarines went on a dive in the late 1980s near the southern Caribbean island of Aruba. Others such as Barbados, Cayman or St Thomas followed.

The submarine Atlantis is based near Charlotte Amalie, the capital of the American Virgin Island of St Thomas. A cruise liner is anchored offshore. Passengers including many families with children watch the swirling water from the ship railings.

A periscope appears, then the tip of the commando tower and the shiny back of the Atlantis. A hatch opens next to the platform and the occupants of the submarine disembark, well tanned and in the best of moods. Then it is the turn of the next group of cruise liner passengers to enter the submarine.

Atlantis starts its dive gradually. At a depth of 15 to 20 metres the coral reef becomes more dense. Numerous marine inhabitants have their home here.

Two children pull their mother closer to the window. "Look they are kissing each other," seven-year-old Carla from Florida calls, pointing to the blue fish known as the Kissing Fish.

At a depth of 30 metres we see the outlines of a shipwreck covered by algae and sea vegetation. Giant tortoises swim around.

"The animals have long become used to the boats and know that they are no danger," says submarine captain Rob Carlin.

But tourists need not travel all the way to the Caribbean to enjoy a submarine trip. Dives are also offered on the Spanish island of Majorca.

"There is a big interest from tourists," says Stuart Dickon, director of Nemo Submarines Baleares.

The Nemo dives into the Mediterranean up to ten times a day. The entire excursion together with a video presentation, transfer to the vessel and the 45-minute dive takes more than two hours. Adults pay a fee of ?50 (about R400) which is about the same price elsewhere in the world.

Submarines also dive near the Canary Islands of Tenerife, Gran Canaria and Lanzarote.

"The demand is huge. More boats have been ordered and are being planned," says Lindsay Laverty of Submarine Safaris in Lanzarote.

For more information visit: www.atlantisadventures.com, www.discoverlanzarote.com, www.nemosub.com.