Submarines are specialized watercraft designed to operate underwater. They are equipped with a streamlined hull to reduce drag and can dive to various depths. Key features include ballast tanks that control buoyancy, allowing the submarine to sink or surface, and periscopes that provide visibility above water while remaining submerged. Modern submarines often have advanced sonar systems for navigation and detecting other vessels. They may also carry torpedoes and missiles for defense or attack purposes. Additionally, submarines are designed with strong materials to withstand high pressure at great depths, ensuring the safety of their crew and equipment.