i know there are other threads on this topic, tho i will respond to this one since there were suggestions of what should there be at the bottom...
1.
how about mimic reality...
a deep deep ocean of lava at the botom. maybe a solid supreme diamond+++ deep deep within that. ^_^
2.
or how about have a substance called turtlerock which takes longer than anything else to destroy ... each block will wear out the strongest tool, (~)and the user's inventory will become full of nothing but 99 turtleblocks (... and even when they are unpicked up, they look like a big turtle, blocking the ability to destroy the block underneath) (~)or just get one turtleblock, but it slows the user down by half. maybe even have the turtleblocks slow the user down by half each deeper layer of it too. and when you smash a turtleblock, you hear a faint echoey ghostly "what's under the turtle blocks?" "it turtles all the way down.".
3.
combination of the above ideas.... (or not)
add a thick slow crust above the lava (/high pressure lava?!!!), that gets hotter and hotter, such that an unprotected player wouldnt last two seconds when close to smashing through. maybe even camerashake and rumbling when about to break through. ... high pressure enough to fill all your tunnels to the surface. ^_^