Actually, some wires don’t require to be placed underground. The major purpose of burying the wire is purely aesthetic. Other reasons might be to protect the wire from any damages or to prevent accidents from happening, such as tripping over it.
However, if you are really planning to bury it underground, you should do so at a depth that is somewhere between 3 and 24 inches. Burying it too deep might lead to some signal strength alterations, or it might come across other buried lines in your yard that are connected to various utilities. Most systems have the required depth written in the manual.