Cute; though this is a case where the mousePressed boolean in draw() is perhaps a better option:

1. //2010 Devan Buggay
2. float[][] ground= new float[250][3];
3. void setup(){
4.   frameRate(60);
5.   size(500,500);
6.   strokeWeight(2);
7. }
8. void draw(){
9.   background(255);
10.   for(int i=1;i<(width/2);i++){
11.     stroke(100,50,50);
12.     line(i*2,ground[i][0]+300,i*2,height);
13.     line(((i-1)*2),ground[i-1][0]+300,i*2,ground[i][0]+300);
14.   }
15.  
16.   if (mousePressed) {
17.     deform(mouseX/2);
18.   }
19.  
20.   println(frameRate);
21. }
22. void deform(float x){
23.     // start the loop from 0 here, otherwise you miss out the first array element
    
24.     for(int i=0;i<(width/2);i++){
25.       int r=abs(int(x)-i);
26.       ground[i][0]+=500/(500+exp(r));
27.     }
28. }

That way the action continues for as long as the mouse is held down; rather than requiring lots of repeated clicking ;)

A version with auto deformation and up and down deformation using mouse.
Would be nice to combine it with example: File / Examples / Topics / Motion / Reflection2.

1. // name: 2D_Terrain_Deformation
2. // auth: 2010 Devan Buggay
3. int xs=500;
4. int ys=100;
5. boolean anim = true;
6. float[] ground= new float[xs];
7. void setup()
8. { frameRate(30);
9.   size(xs, 2*ys);
10.   strokeWeight(2);
11. }
12. void draw()
13. { background(144,155,222);
14.   //smooth();
15.   for(int i=1; i<width; i++)
16.   { stroke(100,80,20);
17.     line(i, ys+ground[i], i, height);
18.   }
19.   if (mousePressed)
20.   { anim = false;
21.     if (mouseButton == LEFT)  deform(mouseX, 1);
22.     if (mouseButton == RIGHT) deform(mouseX, -1);
23.   }
24.   if (anim) deform(random(width), (random(2*ys)-ys)*0.02);
25.   println(frameRate);
26. }
27. void deform(float x, float y)
28. {
29.   for(int i=0; i<width; i++)
30.     ground[i] += y*300/(300+exp(abs(int(x)-i)));
31. }