Mechanical Man said:
It is not possible to modify the terrain during gameplay with triggers, if you did mean this.
The terrian height may be modified in-game using a JASS Script or using WEU.
This Script might work:
function Trig_Terrain_Test_Actions takes nothing returns nothing
call TerrainDeformationCraterBJ(0.5, true, GetRectCenter(GetEntireMapRect()), 512, -64.00)
endfunction
//===========================================================================
function InitTrig_Terrain_Test takes nothing returns nothing
set gg_trg_Terrain_Test = CreateTrigger( )
call TriggerAddAction( gg_trg_Terrain_Test, function Trig_Terrain_Test_Actions )
endfunction
Or simply use the Environment -
Create Terrain Deformation: Crater function of WEU.