Happy Feet has stopped transmitting.
The Emperor Penguin was released into the Southern Ocean eight days ago with a transmitter glued to him, which has been relaying his position back to scientists in New Zealand.
However, not a peep has been heard since Friday.
Sirtrack, which is monitoring Happy Feet's progress, says this indicates the satellite transmitter has not broken the surface of the water since that time.
Sirtrack engineers have confirmed the satellite transmitter was operating as expected at the last transmission received.