Tunnelling Inferno

The earth around you trembles as a 25ft cylinder within range of you that your can see erupts with fire, sending rocks flying out in all directions. Creatures in this zone take 4d6 fire damage as well as an additional 2d8 bludgeoning damage as rocks flow up and out of the earth. Flying creatures caught in the area of effect must make a Dexterity saving throw equal to your spell save DC or be knocked prone onto the ground. Creatures who are more than 10ft beneath the surface don't take any damage.  

The damage of this spell increases when cast at 6th level (5d6 fire damage, 3d8 bludgeoning damage) and 7th level (6d6 fire damage, 3d8 bludgeoning damage).