The Diamond Reserve is the fifth floor of the Deep Caverns.
If you return upwards you will arrive at the Slimehill.
If you continue going downwards you will arrive at the Obsidian Sanctuary.
The following items can be mined in this area:
They are level 15 and have 250 HP. The zombies and the skeletons can be seen holding a diamond sword and bow respectively and wear diamond armor.
The following items are drops from this area:
In the Reserve, there is an area where no mobs will attack you and it is filled full of diamonds. It is right at the entrance to the Obsidian Sanctuary.