Get x and y base on terrain

Hey guys :) I have this grenade that instantiates a explosion, and I have a script attatched to the explosion that sends a message to a script attatched to the terrain, basicly all I want it do is to send a raycast down and see where it hit the terrain and store where it hit the terrain in seperate floats, so that I can use those to dent in the terrain, but I need thos points to use as the base ints for the terrain.setHeights. Is there a way to do this, if so could you please point me in the right direction? Also could someone point me in a direction where I can learn how to make a sort of hemisphere in the terrain with the setHeights? Thanks in advance guys :)