Thor's hammer was forged inside a dying star, that's why it's so damn powerful. But what if it had actually been forged from a neutron star's demise?
The latest
Vsauce video has the answer and it is a friendly one. One sugar cube sized bit of dying neutron star would have the same mass of roughly all of humanity, weighing 400 million metric tons. Thor's hammer would be 4.6 trillion metric tons.
The video puts it, "97 million Titanics condensed into the size of an American football."
Thankfully, Mjolnir wasn't made of all that stuff, otherwise it'd be destroying worlds in an instant.