Harpoons

Harpoons are the ammo for the Harpoon Gun (though they are actually Spears) in Tomb Raider II and III.

Tomb Raider III Harpoons

Harpoons are found in bundles of three or four.

Harpoons can be fired through the Underwater Propulsion Unit in the TR3 level Lud's Gate as well as the Harpoon Gun (though the auto-aim function does not work).

They are generally not very effective in Tomb Raider II, but slightly better in Tomb Raider III.

Every third or fourth shot (TR2 or TR3 respectively) Lara must re-load her Harpoon Gun with some more Harpoons.

In Tomb Raider II, when you have unlimited ammo, this re-loading does not occur (though a small bug may activate in some levels when after one Harpoon shot you have to re-load the gun, making it harder to combat underwater foes with.)