Planets have important things inside them, and a lot of it. I don't know if that planet has the same things as ours does but if it does have layers, how could people displace tons and tons of lava and rock? I'm not sure how deep it goes but how could they mess with the core of the planet, and build a weapon and storage system that big without destroying the planet?
Did Snoke or some other powerful force user aid with the construstion? It doesn't seem logical to me for them to build this weapon.