The present invention concerns a buoyant fluid comprising a liquid and a plurality of rigid containers, the rigid containers each having a sealed void containing a gas. A particular use is to transport heavy objects subsea. The gas in the rigid containers provides buoyancy but does not compress at different subsea pressures. Therefore effective buoyancy control subsea is much easier compared to known methods. For certain embodiments, a secured supply container may be provided subsea to supply the liquid and rigid containers to a lifting device having a container therefor and an attachment mechanism secured to the object being transported. The liquid is preferably a biodegradable substance such as vegetable oil.