diff --git a/projectile.s b/projectile.s
index cb1f7a9..710e86b 100644
--- a/projectile.s
+++ b/projectile.s
@@ -914,7 +914,6 @@ processTerrainImpactStoreContinue:
 	sta PARAML0
 	pha						; Dirt explosion will need this
 	sta compileTerrainRowStart	; Terrain chunk compiler will need this
-	lda #TERRAINWIDTH
 	sta compileTerrainRowEnd
 
 	lda projectileData+GO_POSY,y
@@ -927,6 +926,20 @@ processTerrainImpactStoreContinue:
 	tay
 	PROJECTILETYPEPTR_Y
 
+	sec						; Quick sidebar to calculate crater compiling extents while we have the pointers
+	lda #TERRAINWIDTH
+	sbc compileTerrainRowStart
+	sbc projectileTypes+PT_RADIUS,y
+	sbc projectileTypes+PT_RADIUS,y
+	sta compileTerrainRowStart
+	sec
+	lda #TERRAINWIDTH
+	sbc compileTerrainRowEnd
+	clc
+	adc projectileTypes+PT_RADIUS,y
+	adc #GAMEOBJECTWIDTH
+	sta compileTerrainRowEnd
+;don't forget to clamp these to terrain edges'
 	lda projectileTypes+PT_RADIUS,y
 	tay
 	phy					; We'll need the radius in a moment
diff --git a/terrain.s b/terrain.s
index 34ae3a4..dce122e 100644
--- a/terrain.s
+++ b/terrain.s
@@ -289,14 +289,13 @@ compileTerrainDone:
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ; compileTerrainChunk
 ;
-; Y = First row to compile (bottom-relative)
-; X = Last row to compile (bottom-relative)
 ; compileTerrainRowStart = Start column to compile (logical terrainData index)
 ; compileTerrainRowEnd = Ending column to compile (logical terrainData index)
 ;
 ; Trashes A,Y, SCRATCHL, PARAML0, PARAML1
 ;
 compileTerrainChunk:
+	ldy #0
 	tya			; Be extra safe and make sure Y is never negative or we'll spray RAM with terrain data
 	bmi compileTerrainChunkClampZero
 	ldx #MAXTERRAINHEIGHT-1