How do I cut/sculpt lava rock for my tank?

Hello, I have a 5 gallon tank for my Betta. Inside is a lava rock with some plants on it. It fits in just fine but the tank is round so it looks a bit blown up and out of proportion, All I can see is this giant mountain of Rock in my container and its really starting to bug me. I need to take it out anyway to attach some java moss on, so I started to wonder, all those sculpted rocks in the store have tunnels or holes going through them, could I do it too? I think it would be a really great way to keep the plants and my rock in the tank, and satisfy me in my quest to make it look not so overwhelming. Does anyone know how to cut/knock out a hole through some areas? Can I use a drill, or should I attack it with a chisel and hammer?