Simple but effective way to make it look like the player entered the lava. Just duplicate the lava mesh, and lower the duplicate by a little bit. Then, turn off collision on the original lava mesh. This creates the illusion that the player has sank into the lava. Also neat is that things like the physics boxes for platforms also look like they are submerged in lava (you might have to adjust their size / location since the lava collision is lower now).
