The project, which is about to world premiere in a series of concerts in Rotterdam, The Netherlands, in late May, has taken some time to unfold. Special instruments had to be designed to play under water, such as a carbon fibre violin that won't unglue and a bow with nylon strings, and a specially developed water organ, or hydraulophone, that uses water instead of air to produce sound.

The band's two vocalists also had to train themselves to sing under water, developing the trick of keeping an air bubble in their mouths and singing through the bubble. And because of the way light refracts off the glass in their aquariums, they have to rely on waterproof earphones to hear each other.

But they're not quite mermaids yet -- according to New Scientist, each band member emerges about once a minute to breathe.