|Another alternative would be a point-to-point wireless shot. This would require two antenna's.|
I've worked with equipment from EnGenius and found it reliable and easy to configure. An example of what I'm talking about can be seen on the following link:
If you really want to go with the buried cable you will want outdoor grade cable. It has a heavier jacketing and will have grease inside it too. You would also want to use some PVC pipe as conduit between buildings to pull the cable through. I would recommend using minimum Cat6 cable.
Once run you would then terminate and test the cable and when it's working, plug it into the AP. You would want to configure and test the AP before installing it.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.