In this node I aim to describe a less known method for lifting objects of all sizes from the deep sea. I'll mention some of its advantages and use comparisons with well known methods for demonstration.

The Concept

Imagine if you will, a hot air balloon. The balloon floats through the atmosphere by exploiting the fact that the warm air it contains is slightly less dense than the surrounding air. The pilot can control the altitude of the balloon through the implementation of a large gas burner, which further heats the air inside, further reduces its density and thus creates more lift.

Now imagine the balloon is no longer filled with air, but with fresh water. Picture it travelling not through the atmosphere, but beneath the ocean. Fresh water is slightly less dense than salt water, giving our submerged balloon lift. The balloon is then connected to a load using short pieces of conventional cable. The controller can increase this lift by pumping more fresh water into the ballon from a surface vessel. However, the lift can also be reduced by operating a valve at the top of the balloon, which lets fresh water out.

OK, so by this point you may be starting to wonder why anyone would bother with an underwater balloon for deep sea salvage. The basics sound ok, but where's the advantage over other, better known methods? Allow me to describe some of the problems of three of the traditional lifting methods and all will become clear:

1: Air bags

Air bags make use of the same premise, using something that floats in water to drag your salvage upwards. The problem lies in the compressibility of air. At depth the air in the bags will crushed by the water pressure until it is almost as dense as the water around it. However, once lift is acheived the water pressure reduces and the air can expand inside the bags. The resulting drop in the density of the air creates more lift and the air bags and the salvage accelerate upwards. This can become dangerous because the rate of climb will continue to increase as the rig nears the surface.

It's been known for a piece of salvage, to be lost when the air bags pulling it leap clean out of the water.

Compare this to the fresh water lifting method. Water is virtually incompressible, so as the balloon rises in the salt water sea it does not experience anything like the same rushing acceleration of an air bag. This allows plenty of time to throw away any surplus lift using the outlet valve.

2: Oil Bags

Oil bags have been used in deep sea salvage for as long as the industry could be called such. They overcome the major problem with air bags that I have described above, however they still have a serious flaw: pollution. In years gone by people were happier to spill a bit of oil in the sea, but in modern times education as to the effects on marine life has made this less acceptable.

If an oil filled balloon ruptures you lose your salvage and you have a small scale environmental disaster on your hands. If a balloon full of fresh water bursts you also lose your cargo, but this time the impact on the local marine environment is roughly equivalent to a light shower of rain.

3: Cables

Cables are the most straight forward tool for lifting salvage from relatively shallow depths. Unfortunately their downfall becomes apparent when you are lifting in deep seas. The trouble is that metal cables are heavy. Imagine how much a five kilometre long, steel cable thick enough to lift a 20,000 tonne submarine weighs. Beyond a certain depth a metal cable will snap under its own weight.

So what about using a fresh water balloon? Surely the pipe that's used to pump water down to the balloon will also suffer the same problems? There are two reasons why this is not the case. Firstly, the pipe is not being used to lift the salvage because that's done by the balloon, so it doesn't need to be as strong as a metal lifting cable would. This means it can be made of lighter materials. Secondly the pipe itself will always be full of fresh water. This water also provides a certain amount of lift, causing the pipe to be almost self supporting in the salty sea water.

Bottom Suction: A Common Problem

Bear with me here because this sounds like an excuse for double entendres...

The phenomenon of bottom suction in deep sea salvage operations is the result of lifting an object out of sand, mud, silt or the like. When a heavy object sinks to the sea bed it can be travelling quite fast and its momentum will cause it to dig in, creating a crater much like a meteorite striking land. Over time the local currents will fill this crater in and silt may become piled up around the sunken object.

The result is that the salvage can become stuck into the sea bed. This means that the force required to start lifting it can be a lot greater than if it were simply lying on a smooth, flat surface. The problem then becomes one of getting the lift back under control. If air bags are being used you can find you have a lot of surplus acceleration to throw away. If you are using cables you can find that they stretched under the force required to move the salvage, but now the load on them is reduced they will rapidly contract, which again can cause a problematic surplus of acceleration*.

The use of fresh water balloons reduces the severity of this problem because they are less prone to sudden movement and this means there is a lot more time to control them.


I have so far concentrated on problems and solutions with lifting objects from the deep ocean floor, but before I conclude, I am going to turn this on its head and tell you how fresh water balloons can be invaluable to those concentrating on placement.

Deep sea operations are often based around lifting treasures of economic or historical value or may even be investigatory, simply the result of people wanting to know what went wrong and caused a vessel to sink. However, many operations are now setup to place objects onto the ocean floor. These may be naval monitoring stations, placed in strategic points where they can listen for passing submarine traffic. They may be seismic monitoring devices, used by the oil and gas mining industries. They may even be pieces of oil pipelines or data cables.

The fresh water balloon's usefulness becomes readily apparent in this type of work. It is easy to raise or lower the load to the correct depth. It can even be so finely controlled as to hold an object steady while a diver or robot welds it in place. However, the main boon is that the balloon does not require a fixed length connection to the surface. The water hose can be allowed to be longer than the mean depth of the load, allowing a surface vessel to be thrown around by large waves without it pulling the load all over the place down below.


To sum up, deep sea salvage, exploration and construction can be a costly and dangerous exercise. We have so much to learn of the deepest oceans, but do know the whereabouts of masses of sunken relics, lost gold carrying Nazi U-boats, even Spanish galleons that sank on the way back from ripping off the Incas. So while there's so much to attract people to the depths we are always going to need good ways to access them. What better than using fresh water balloons that are controllable, cheap and harmless to the environment?


  • This salvage method is patented worldwide by Controlled Lifting International.
  • You need 50 times as much fresh water as the weight you are lifting (2% density difference) in most oceans.
  • Less is required in the Mediterranean, because it's saltier, and much less in the Dead Sea.

* I'd like you to read about Heave Compensation at this point, please.

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