Does impassable terrain block LOS of massive models?

I think so - I just want to be sure.

Impassable terrain doesn't actually block LOS for anyone. You presumably mean blocking terrain which does unless the massive figure is actually on it (see the FAQ).

Yes we meant blocking terrain.

If a massive figure in on a blocking terrain, does this ruling only appeal on the acutal blocking terrain unter the figure or for all blocking terrains on the map?

I'm pretty confident (but not entirely) that it's only the terrain under the figure.