what could cause this?

i have made a few meshs recently, one is a terrain for someone, the other is the interior of a starship. when i build the level to take a look at the model i can see certain parts through other parts, like i can see through montains and walls. is this a mesh problem or perhaps a scripting one? has anyone ever had this trouble before?

you used a tga with alpha-chanel for it.
Look at the wiki there is a alpha-sort material to avoid this effect

It's also possible that you have the normals flipped on some parts of the model.
You can check this in MED.

seems a bit nuts that i have to use a material if i`m useing an alpha channel, so to save doing that i have booleened the portholes instead of useing an alpha channel, cheers guys

just do a fast check in MED:
select all faces, flip normals and check what happens in your level.
If you see now through the other parts, but the original holes are "solid" now, then you have a face orientation problem. In this case just flip the problematic faces.

it`s not a face problem, i made the mesh in max and had no need to flip the faces on a terrain

Sometimes it is an exporting problem. I have seen several cases, where the model in max, blender or C4d looks perfect, but during exporting to mdl some faces got reverted.
Do you have the same problem, when you preview it in MED?

I haven't checked in the current version to see if this has been fixed or not, but in the last release, if you changed the location of a texture in MED's texture settings (for example, you imported with the model's own textures, but then decided after import to replace one with an texture in an external file) and applied it to the model, the normals would invert for all parts of the model using that texture.

cheers galen i`ll check on that