Hardened Stone

Currently, apart from ores and lava, there is no distinction between the higher and lower underground. A way this could be improved is by giving stone a higher hardness and blast resistance the deeper underground you go. This would also add some difficulty to mining, as you'd want to mine lower down for diamonds etc. but you would have to contend with harder stone. Also, this would give the Efficiency enchantment more use as it would massively improve the experience underground and encourage players to invest in enchantments.