That said…
St Bonney Bonn, I dug a mineshaft under my house and eventually hit a huge system of caves that eventually led back to that spider spawner under your house. If you decide to go looking for natural coal deposits again, I saw several big veins of it through the caves. I lit up the place like a christmas tree; so, you shouldn't have to worry much about hostiles.
Also, I built a "test chamber" underground and confirmed that smooth stone and brick seem to be equally affected by explosions despite brick being harder to break down with a pick. So, brick might slow down griefers, but it won't slow down creepers!
edit: Wow. These caves are bigger than I thought. It looks like I've only lit up maybe half of the whole system.