you mean craters ?
you can simply make those with this easy
get 4 triangular brushes
line them up like this
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|\ /|
| \/ |
| /\ |
|/ \|
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*note* imagen this looking like a blok and the name of the thingy's are wedges
then use the vertix manipulate tool on 1 of them
(make sure snap to grid is on )
drag the center ( or center 2 ) points down ( i nealy never use the yellow ones as they are hard to allign to a 'big' grid)
now do the same to the 3 other ones (so there are no gaps left ) and youll have a crater !
this is the well best way i gues to make nice looking terrain