Image caption Deep Ocean Technology has designed its rooms to minimise noise on the sea floor

When it comes to luxury hotel rooms, down is becoming the new up.

Instead of penthouse suites, the most exclusive accommodation in the house may soon be at the very bottom, 20ft (6m) or more beneath the surface of the sea.

The idea of sleeping with the fish some distance beneath the waves may sound dangerous but the technology needed to build underwater rooms is well-established and proven.

So says Robert Bursiewicz, a project manager at Deep Ocean Technology, a Polish company that is planning an underwater hotel called Water Discus. It can be towed to a suitable location and placed on supports on the sea bed.

It is made up of an underwater disc containing 22 bedrooms with sea views, connected by lift and stairs to a similar disc above the surface containing other hotel facilities.

"Nowadays it is possible to build submarines which go deeper than 500m (1,640ft) below the sea surface, so building an underwater hotel is not a problem," Mr Bursiewicz says.

Now that technology is being used in a new way, he says.

Image caption The technology to keep water out of the rooms has already been tested - for example in submarines

In fact, it should be far less of a technical challenge to build an underwater hotel than a submarine, because hotels are unlikely to be placed more than 15m (50ft) below the surface.

That is because the water acts like a filter for sunlight, and below 15m most colours apart from blue are washed out.

And that means rooms need to be in relatively shallow water for the most spectacular and colourful views of marine life.

'Submarine or oil rig'

It would be very exciting to have an underwater hotel here, but perhaps our cold and tidal seas are better for surfing and sailing Tabitha Aldrich-Smith, British Hospitality Association

"We are aiming to have rooms at a depth of about 10m (33ft), as that provides a good colour environment in sunlight," Mr Bursiewicz says.

A bigger potential challenge than keeping the water out is finding a way to keep in the inevitable noise that an underwater hotel generates.

That is important because a noisy environment is likely to disturb and scare away fish and other marine creatures - rather defeating the purpose of building an underwater hotel in the first place.

Mr Bursiewicz says that this problem has been overcome by careful design from the beginning, ensuring that items like lavatories, pumps and air conditioning equipment that generate noise are placed at the centre of the underwater structure.

It is also important that an underwater hotel complies with local laws and regulations, and that can be complicated because it is difficult to know which ones apply to this type of construction.

"Every country has different marine regulations for different things, " says Mr Bursiewicz.

"It's difficult because you could think of our hotel as a kind of marine device [like a submarine], or as a ship, or perhaps as an offshore construction like an oil rig."

Image caption Jules' Undersea Lodge has hosted guests off the Florida coast since 1986

Better for surfing?

Finding a site where an underwater hotel could legally be positioned can also be hard.

One could be built within 12 nautical miles of Britain's coastline, but only with permission from the government and as long as it did not infringe on international laws governing things like the protection and preservation of the marine environment, according to Dr Alexandros Ntovas, an expert in maritime law at the University of Southampton.

We have to put everything in waterproof boxes and attach weights to them to counter their buoyancy Teresa McKinna, Jules' Undersea Lodge

But there are currently no plans to build in British waters.

Tabitha Aldrich-Smith, spokeswoman at the British Hospitality Association, believes that it is unlikely that one will ever be built here.

"It would be very exciting to have an underwater hotel here, but perhaps our cold and tidal seas are better for surfing and sailing on rather than staying underneath," she says.

The warmer oceans of the south-east United States are much more suitable and one hotel - the two-bedroom Jules' Undersea Lodge, off the coast of Florida - has been taking in guests 31ft beneath the surface since 1986.

The construction was previously used in the 1970s as a marine laboratory under the sea off the coast of Puerto Rico.

Image caption Guests must get into scuba gear and dive down to enter the undersea lodge

Jules differs from Deep Sea Technology's proposed water discus hotel in that the entire structure is underwater, accessible only by swimming down to it using scuba gear and entering through an opening at the bottom.

The company that operates the hotel also runs a three-hour scuba course for beginners, which covers enough to get guests down and in to the hotel.

The only requirement is that they can swim.

Teresa McKinna, Jules's chief financial officer, says that the lodge is equipped with a television, air conditioning and wi-fi.

It is also safety-checked every day by divers, and is equipped - somewhat ironically given its location - with fire extinguishers.

Ms McKinna says that the pressure in the hotel is the same as on the surface, so there is no limit to the length of time guests can stay.

Despite this, guests should not dive or fly for 24 hours after staying overnight.

Image caption The world's newest underwater room is at the Manta Resort in Tanzania

'Endless wear and tear'

From a hotel management perspective, Ms McKinna says that simple things like cleaning and changing bed linen can be challenging, since the lodge is submerged in sea water.

It is a matter of mathematics. There are lots of parameters that you have to calculate, but you have to do that with any hotel Mikael Genberg, Manta Resort

"It actually takes quite a lot of work to get anything down there," she says. "We have to put everything in waterproof boxes and attach weights to them to counter their buoyancy."

The hotel has suitably sized waterproof boxes, so that pizzas can be delivered by divers to guests who order them for their evening meal, she adds.

The latest underwater hotel room to be unveiled is at the Manta Resort in Tanzania, off the coast of Pemba Island.

It is unusual because, unlike Water Discus or Jules' Undersea Lodge, it is not supported by the ocean floor.

Instead, it is attached to a structure that floats on the surface of the sea, and which is anchored to the sea floor like a boat to prevent it drifting off.

Guests enter the hotel room by going down a flight of stairs from the floating structure.

Image caption Because of strong tides and currents, the structure floats on the surface rather than sits on the sea floor

Mikael Genberg, its designer, explains that the reason for this was safety.

"Off Pemba Island there are 4m (13ft) tides," he says. "That means there are currents, and there would be endless wear and tear on a structure attached to the sea floor.

"For a hotel room, the danger would be quite extreme and sooner or later something would break. That is why we have built something that is attached to the surface."

Once the decision had been made to attach the room to what is, in effect, a boat, designing it was fairly straightforward, he concludes.

"Like everything, it is a matter of mathematics. There are lots of parameters that you have to calculate, but you have to do that with any hotel that goes up.

"The only difference is that in our case we are going down."