maps mosaic

[Piero Cavalieri]

Hi all,
I'm new to WebGIS, and in general I used GIS not too much.
The question is:

I have a lots of DTM which toghter cover a wide area, wich I want to have
in the extents of my .map

I want that these DTMs act as a 1 layer in the map.

Have I first to create a single map from these DTMs with a DesktopGIS (e.g.
GRASS) or there is a method to tell MapServer that all that DTMs have to
act as single layer ?

Thanks
    Piero