Gnomish Submarine (Warcraft II)

The resourceful Gnomish Inventors have designed an amazing craft known as the Submarine. This watertight vessel can submerge beneath the waves and surreptitiously keep watch on enemy forces above the surface. The Gnomish Submarine is visible only to Towers, aerial forces, and other vessels that move beneath the waves. Using cunning and craft to carry out surprise attacks on more powerful enemies makes the Submarine an invaluable part of the Lordaeron armada.

• Submarines are special naval units that are invisible to all enemy units except fliers, Towers, and Giant Turtles. They are used to destroy enemy sea buildings and any unescorted ships that cannot see them. They cannot attack enemy land units or buildings.
• Even if they cannot see them, enemy units can still destroy Submarines by using the "Attack Ground" command or certain spells.
• Submarines are mostly useful when the enemy is focused on an early-game rush and therefore puts off building a Goblin Alchemist. It is not prudent to build them once Zeppelins become common.

• See Warcraft II gnomish submarine quotes