Your Vehicle should be able to withstand extremely cold temperatures, seeing you'll be very low below ground, seeing that there is molten lava around the core, you need to withstand major heat, also, graphene would be the best material for your Vehicle, because it is the strongest material known to man. There are also about 8 layers of earth, Crust, Moho, Upper Mantle, D"- layer, Outer Core, Liquid/Solid Boundary, Then the inner core! the Liquid/solid would be extremely hard to get through, so beware.