This block is the only block that can spread, it only spreads across and downward, all lava blocks are counted as source blocks meaning that any lava block can make more lava blocks. It kills any player instantly making this a very hazardous block when mining, it won't burn through any block, it is stopped by the force field block which allows it to be used in traps and in decorative designs. It is for the most part found in very low down in the world, it may not be mined and blocks may not be placed on or in it, the only way to remove is with TNT.
This is also the only liquid in the game.
The lava flowing from the top isn't whats creating the lava down below, when the top part is destroyed what is left of the flow will stay there much as if it were just normal blocks. Most lava pockets will not be created like this as most are just lakes, some lakes spawn in open air due to a cave being generated, it has yet to be seen to see a lava lake completely encased in Metal or Stone or any other block.
Surface lava pools are quite rare but possible, this makes having a base under lava quite conveinent.