map 2d coordinates to 3d coordinates in the scene

Does anybody know how to map a 2D coordinates for a window to a 3D coordinates for the scene drawn in that window?

That is:when user click mouse in a display window,the position in the window can be easily found by callback defined in glutMouseFunc(),but how can I find out the corresponding 3d coordinates in the scene ?


Surely that point could be anyware along the Z axis.