The Visual 6502 group enjoys dissolving old chips in acid, photographing the die, and reverse-engineering them.
Unfortunately (or not, depending on your perspective) it's not just the Visual 6502 group that enjoys that. Any leading chip in the marketplace these days is almost immediately reverse engineered in this way. I haven't been in the chip business for a few years but I recall a company which offered this as a complete service - they'd send you detailed circuit schematics and so forth.
The difference is the Visual 6502 team does it for fun. It makes me wonder what cutting-edge technology from today will be hobby projects 40 years from now. My predictions: scanning tunneling microscopy, lots of genomics / molecular biology, "big data", some current cryptography, a lot of AI.
And to reply to the parent comment - designing the calculator in the first place is much more amazing.
If you go back through their blog posts, they have some cool techniques for bypassing "security" metal meshes to read out the secure FLASH/ROM sections of chips.
IIRC, one of the guys behind FlyLogic was involved in the high-profile reverse engineering of satellite receivers (ie. satellite pirating).
Unfortunately (or not, depending on your perspective) it's not just the Visual 6502 group that enjoys that. Any leading chip in the marketplace these days is almost immediately reverse engineered in this way. I haven't been in the chip business for a few years but I recall a company which offered this as a complete service - they'd send you detailed circuit schematics and so forth.