Digging Bolt

Digging Bolt is a projectile spell that creates a short-lived spherical area at the point of casting which is capable of digging through materials.

Digging Bolt can dig through materials with a durability of 10 and under, meaning it can dig through the 10 durability Dense Rock but not the 12 durability Extremely Dense Rock.

It is not very effective for combat, but can be used in a pinch.

Tips

Digging Bolt is one of only a few projectiles capable of delivering Drill damage. The other primarily being Digging Blast.
A really useful tool if combined with the right wand with sufficiently low cast delay and, to a lesser extent, recharge time (for example the second starting wand).
It can be used to dig through soft ground very easily, and to get gold really fast. Hundreds of gold can be fairly easily amassed without any real danger, especially in the second biome on the main path, Coal Pits.
If used with the Increase Duration projectile modifier, the spell will "drip" white balls, digging in the ground beneath the cast area.
Just like Chainsaw, Digging Bolt can be used to reduce the recharge time of any wand in which it is cast, though with slightly less efficiency due to its small positive modifier to cast delay.
Along with Digging Blast, this spell has the property of overriding any other sounds generated by the spell. Because of this, adding Explosive Projectile will cause the explosion sound not to play. This is particularly useful when making endgame teleport wands in order to avoid constant loud sounds.
Be careful when increasing Digging Bolt's explosion radius, as it will produce sparks which are capable of igniting you.

Trivia

When Increase Lifetime is applied, the projectile will visibly fall downward. It only applies the digging effect at the end, and by default dies on collision with enemies. Having too many copies out generates a large amount of lag.
Only collides with "hittable" entities. This excludes, e.g., Pollen projectiles.
Combined with Glimmer modifiers, its sparks will be turned into various real materials that can interact with the world. Blue Glimmer can be used to produce and bottle Magical Liquid, while the other possible materials can't be bottled directly from the world^[1].