The robotic space craft Beagle 2 touched down on Mars on Christmas Day 2003, but it never sent back a radio signal to say that it had survived the landing.
It may have been destroyed in a crash or had a computer or battery power problem.
Scientists tried really hard to contact the probe, but they never found out what went wrong.
Beagle 2 travelled to Mars aboard its mothership, Mars Express. It was released and descended to the red planet's surface.
The aim of its mission was to look for life on Mars, and see how the mountains and rocks formed.