A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. Submarines are referred to as “boats” rather than “ships” irrespective of their size. Submarines can work at greater depths than are survivable or practical for human divers and can remain submerged for months. Submarines were first widely used during World War I (1914-1918) and are now used in many navies large and small.
Military uses include attacking enemy surface ships (merchant and military), or other submarines, aircraft carrier protection, blockade running, ballistic missile submarines as part of a nuclear strike force, reconnaissance, conventional land attack (for example using a cruise missile), and covert insertion of special forces. Civilian uses for submarines include marine science, salvage, exploration, facility inspection and maintenance, undersea cable repair, tourism and undersea archaeology.
Here are the top 10 largest submarines in the world.
| Rank | Submarine | Country | Submerged displacement |
| 1. | Typhoon-class submarine | Russia | 48,000 tonnes |
| 2. | Borei-class submarine | Russia | 24,000 tonnes |
| 3. | Oscar II-class submarine | Russia | 24,000 tonnes |
| 4. | Ohio-class submarine | United States | 18,750 tonnes |
| 5. | Delta-class submarine | Russia | 18,200 tonnes |
| 6. | Vanguard-class submarine | United Kingdom | 15,900 tonnes |
| 7. | Triomphant-class submarine | France | 14,335 tonnes |
| 8. | Akula-class submarine | Russia | 13,800 tonnes |
| 9. | Yasen-class submarine | Russia | 13,800 tonnes |
| 10. | Sierra-class submarine | Russia | 10,400 tonnes |
