Casu marzu is a Sardinian black-market cheese with what rather surprising ingredient?

The surprising ingredient in Casu Marzu is live insect larvae. Specifically, the larvae of the cheese fly (Piophila casei).

These larvae break down the cheese, making it softer and spicier. The cheese is considered a delicacy by some, though it is illegal to produce and sell in most of the world due to safety concerns.