Relativity "unleashed" to us the energy and mass can be converted. Nothing is ever gained or lost. If you burn an egg and collect all the ashes and heat and light and weigh the findings, it would add up to the weight of the egg. If you take enough mass in a relatively small place in space. It will press down (but you have to think in a 3D sense) hard enough that nothing can climb of that hill and get out.