Sailors, fishermen and cruise passengers should be on the alert. If anybody spots a yellow rubber duck bobbing on the ocean waves, Nasa would like to know.

The US space agency has yet to find any trace of 90 bathtub toys that were dropped through holes in Greenland's ice three months ago in an effort to track the way the Arctic icecap is melting. Scientists threw the ducks into tubular holes known as "moulins" in the Jakobshavn glacier on Greenland's west coast, hoping they would find their way into channels beneath the hard-packed surface, to track the flow of melt water into the ocean.

"We haven't heard anything from them yet," Nasa robotics expert Alberto Behar told the BBC.

Also missing is a football-sized floating robotic probe equipped with a GPS positioning transmitter and powered by hi-tech batteries. It has failed to communicate its position. "We did not hear a signal back, so it probably got stuck under the ice somewhere," said Behar.

The experiment was intended to examine the movement of glaciers, which has speeded up in recent years. Scientists believe that melting water lubricates the bases of glaciers.

Although low-tech, the $2 ducks were chosen for their buoyancy and for their ability to withstand low temperatures. Nasa is offering a modest prize of $100 to the first person who finds a duck. The ducks have an email address stamped on them, together with the word "reward" in three languages, including Inuit.