Can someone explain why and how to count nested arrays

Can someone explain why and how to count nested arrays. meaning why is it [3] [4] [3] [0]…

$treasure_hunt = [
  "garbage",     //  0
  "cat",         //  1
  99,            //  2
  [              //  [3]
    "soda can",     //  0
    8,              //  1
    ":)",           //  2
    "sludge",       //  3
    [               //  [4]
      "stuff",         //  0
      "lint",          //  1
      [                //  [2]
        "GOLD!"           //  [0]
      ], 
      "cave", 
      "bat", 
      "scorpion"
    ], 
    "rock"
  ], 
  "glitter", 
  "moonlight", 
  2.11
];

Study the above expansion of the nested array. Note the lines with, // [...]