Models under terrain level

Models under terrain level

in the map Azeroth Grand Prix, you can see some Naga Ramps under the terrain's level. How can I do it? It's not in the object's properties, because when you move it, it turns normal again.

It only works with doodads

Hold "Ctrl" and press either PgUp or PgDn depending on what direction you want it to go in on the Z axis

I can't believe that this is so easy... thanks Mecheon