This is under the ocean. I found this while digging a horizontal, 2x2, North-South tunnel from the large island to the NE desert (X axis 294-295). I was working from bedrock level (Y:4). It is possible to break into lava on this path by gravel falling from the ceiling after the stone under it has been removed. There is some lava above due south of the diamonds. There is some good gold in this tunnel also, but I didn't map it.
Also, if anyone else is in dire need of silk touch like I was and couldn't find it in the mineshaft, I found mine in the chest buried next to the above ground Zombie spawner previously mentioned at x277 y66, z-265.