By Dr David Whitehouse BBC News Online science editor

What lurks beneath the clouds?

The giant Arecibo radio telescope in Puerto Rico has transmitted a beam of radio waves towards Titan, and detected a faint echo over two hours later.

Analysis of the dim signal suggests the presence of craters filled with oily oceans or lakes beneath the clouds.

In January 2005 a European Space Agency probe - Huygens - will parachute on to Titan's surface to see what is there.

Down to a sunless sea

Titan is one of the most intriguing and significant bodies in the Solar System.

Arecibo sent out the signals

Optical observations cannot see through the photochemical smog that shrouds the world, but infrared and radar radiation can get through, revealing a varied surface beneath the clouds.

Ground-based telescopes and the Hubble Space Telescope have produced coarse maps of the surface, showing what could be a continent of rock and ice surrounded by hydrocarbon seas or lakes.

Hydrocarbons - methane and ethane - could form oily oceans on the surface - whose waves lap against shorelines of ice stained by hydrocarbon drizzle from the sky.

The Arecibo signals took 2 hours and 15 minutes to return. A tiny fraction of the transmitted energy was detected at Arecibo as well as at the Green Bank radio telescope in the US.

Will it splash down?

As expected, the echo contained a broad diffuse component. In most cases it also had a sharper signal just like that expected from a broad flat region like the surface of an ocean.

Confirmation will come next year when the Cassini space probe reaches the Saturnian system and begins a series of close flybys of Titan.

In January 2005 Cassini will drop the Huygens probe on to Titan, which may land with quite a splash.