Smuggler's Key
From WikiRaider
The Smuggler's Key is a key item found in the Coastal Village level in Tomb Raider III. It unlocks a trapdoor inside a hut through which Lara can reach an underground cave. If she does so, she can obtain the Harpoon Gun.
The smuggler's key is optional. Lara can also take the route through the swamp instead of going through the cave.