Why is my script causing Unity to crash?

I am working on a voxel role-playing game, and I wrote a script to randomly generate a world. I will put the script at the bottom. I created a prefab for the voxel, and it is just a cube with nothing attached to it. Now, when I attach the script to an empty GameObject, and hit play, Unity hangs forever until you end the process with Task Manager. There does not appear to be a memory leak in my code. Why is this happening?

 using UnityEngine;
 using System.Collections;
 
 public class WorldGenerator : MonoBehaviour
 {
     public System.Random rand;
     public int WORLD_HEIGHT = 64;
     public int WORLD_LENGTH = 64;
 
     private double CUBE_SIDE = 0.5;
 
     private float[,] grid;
     private int[, ,] blocks;
     public float roughness = 0.1f;
 
     public Transform Cube;
 
     // Use this for initialization
     void Start()
     {
         rand = new System.Random();
 
         int xh = WORLD_LENGTH - 1;
         int yh = WORLD_LENGTH - 1;
 
         grid = new float[WORLD_LENGTH, WORLD_LENGTH];
 
         grid[0, 0] = (float)rand.NextDouble() - 0.5f;
         grid[0, yh] = (float)rand.NextDouble() - 0.5f;
         grid[xh, 0] = (float)rand.NextDouble() - 0.5f;
         grid[xh, yh] = (float)rand.NextDouble() - 0.5f;
 
         Gen(0, 0, xh, yh);
 
         blocks = new int[WORLD_LENGTH, WORLD_HEIGHT, WORLD_LENGTH];
 
         for (int x = 0; x < WORLD_LENGTH; x++)
         {
             for (int z = 0; z < WORLD_LENGTH; z++)
             {
                 int height = (int)System.Math.Floor((double)(grid[x, z] * WORLD_HEIGHT));
                 for (int y = 0; y < WORLD_HEIGHT; y++)
                 {
                     if (y <= height)
                     {
                         blocks[x, y, z] = 1;
                         Vector3 blockpos = new Vector3((float)(x * CUBE_SIDE), (float)(y * CUBE_SIDE), (float)(z * CUBE_SIDE));
                         Instantiate(Cube, blockpos, Quaternion.identity);
                     }
                     else
                     {
                         blocks[x, y, z] = 0;
                     }
                 }
             }
         }
     }
 
     private void Gen(int xl, int yl, int xh, int yh)
     {
         int xm = (xl + xh) / 2;
         int ym = (yl + yh) / 2;
 
         if ((xl == xm) && (yl == ym)) return;
 
         grid[xm, yl] = 0.5f * (grid[xl, yl] + grid[xh, yl]);
         grid[xm, yh] = 0.5f * (grid[xl, yh] + grid[xh, yh]);
         grid[xl, ym] = 0.5f * (grid[xl, yl] + grid[xl, yh]);
         grid[xh, ym] = 0.5f * (grid[xh, yl] + grid[xh, yh]);
 
         float v = Roughen(0.5f * (grid[xm, yl] + grid[xm, yh]), xl + yl, yh + xh);
 
         grid[xm, ym] = v;
         grid[xm, yl] = Roughen(grid[xm, yl], xl, xh);
         grid[xm, yh] = Roughen(grid[xm, yh], xl, xh);
         grid[xl, ym] = Roughen(grid[xl, ym], yl, yh);
         grid[xh, ym] = Roughen(grid[xh, ym], yl, xh);
 
         Gen(xl, yl, xm, ym);
         Gen(xm, yl, xh, ym);
         Gen(xl, ym, xm, yh);
         Gen(xm, ym, xh, yh);
     }
 
     private float Roughen(float v, int l, int h)
     {
         return v + roughness * (float)(rand.NextDouble() * (h - l));
     }
 }