Currently I've been using a 3D array for my tiles in a 2D world but the 3D side comes in when moving down into caves and whatnot. Now this is not memory efficient and I switched over to a 2D array and can now have much larger maps. The only issue I'm having now is that it seems that my tiles cannot occupy the same space as a tile on the same z level. My current structure means that each block has its own z variable. This is what it used to look like:

map.blockData[x][y][z] = new Block();

however now it works like this

map.blockData[x][y] = new Block(z);

I'm not sure why but if I decide to use the same space on say the floor below it wont allow me to.

Does anyone have any ideas on how I can add a z-axis to my 2D array?

I'm using java but I reckon the concept carries across different languages.

Edit: As Will posted, RLE sounds like the best method for achieving a fast 3D array. However I'm struggling to understand how I would even start to implement it? Would I create a 4D array the 4th being something which controls how many to skip? Or would the x-axis simply change altogether and have large gaps in between - for example [5][y][z] would skip 5 tiles? Is there something really obvious here which I am missing?

The number of z levels I'm trying to have is around 66, it would be preferably that I can have up to or more than 1000 in x and y.