When California houseboat residents heard their low, submarine hum in the 1980s, they thought it might be coming from noisy sewage pumps, military experiments or even extraterrestrials.

But this was the nocturnal hum of the midshipman fish; a courtship call, and the source of a biological secret scientists have now solved.

Researchers brought the fish into their lab to work out why they sang at night.

The US team's findings are published in the journal Current Biology.