The Soviet Union/Russian Navy Project 705 (Лира/Lira, "Lyre") was a class of hunter/killer nuclear powered submarines. The class is also known by the NATO reporting name of Alfa. They were the fastest and deepest diving class of military submarines built, with only the prototype K-278 Komsomolets (NATO "Mike" class) exceeding in crush depth and the prototype K-222 (NATO "Papa" class) exceeding in submerged speed.

The Alfa was a unique design among submarines; it used a powerful lead cooled fast reactor as a power source, which greatly reduced the size of the reactor compared to conventional designs, thus reducing the overall size of the submarine, and allowing for very high speeds. However, it also meant that the reactor had a short lifetime and had to be kept warm when it was not being used. As a result, the Alfas were used as interceptors, mostly kept in port ready for a high-speed dash into the North Atlantic.

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