To build dense abs with deeply etched lines, you'll need to subject them to some hypertrophy training. Hundreds of bodyweight crunches won't give you "cut" abs. You need to start adding some weight. A cable is one of your best options. BOSU cable crunches give you a reason to pick that thing up and start using it.
BOSU Cable Crunch
The BOSU ball's shape allows you to load your rectus abdominis (six-pack muscle) through a greater range of motion.
You can also change the cable's height to emphasize certain points of the crunch. A lower cable will load your abs in their stretched position more, while a higher cable will emphasize the shortened (squeezed) position at the top of the crunch.