The first safes were made from metal. So the first safecrackers simply drilled in using a drill bit made of a harder metal. (Setting aside the options of cracking the combination or blowing the hinges off.)
So the safemakers started using really hard metal – a ‘hardplate’ – eventually settling on cobalt plating, particularly right around the dial and the mechanisms that stop the lock bolt from moving.
So the safecrackers started using diamond-tipped drill bits. That will go through cobalt eventually, although it may burn out the drill motor first.
So the safemakers started using tricks like this: They make a hardplate of a cobalt alloy and sprinkle it with tungsten carbide chips, which can shatter the diamond-tipped drill bits.
So the safecrackers also started trying torches to melt the alloy instead of conventional drills. ‘Thermal lance’ is a cool-sounding safecracking tool.
So the safemakers started adding other layers of material besides the cobalt. Fire-resistent materials to thwart the thermal lance. Spongy stuff or angled soft-steel plates in concrete to tangle drill bits instead of stopping them cold. They also came up with the concept of “relockers”. Typically if you trigger the relocker, a new set of bolts spring into place. Ironically the trigger mechanism in some cases is a plate of glass, which obviously would get broken if you drilled where you were expecting to hit cobalt hardplate.
And so on.