Coccolithophores are microscopic algae that first appeared 220 million years ago, and flourished during the cretaceous period. They produce peculiar plates called cocoliths out of calcium carbonate, and incorporate them into an external shell. They constantly remove carbon from the atmosphere as they die and sink to the ocean floor, producing chalk. This is an important feedback system in the global carbon cycle.
Seriously, how does cellular machinery produce these structures? Life is amazing.