When you discard this other than during a Clean-up phase, you may reveal it. If you do, gain a Gold.
This is both a Victory card and a Reaction. At the end of the game, Tunnel is worth 2VP Tunnel's Reaction ability functions when you discard it. You cannot simply choose to discard it; something has to let you or make you discard it. This ability functions whether you discard Tunnel on your own turn (such as due to Oasis) or on someone else's (such as due to Margrave). It functions if Tunnel is discarded from your hand (such as due to Oasis) or from your deck, or when set aside (such as due to Cartographer). If Tunnel would normally not necessarily be revealed (such as when discarding multiple cards to Cartographer), you have to reveal it to get the Gold. Revealing it is optional, even if Tunnel was already revealed for some other reason; you are not forced to gain a Gold. This ability does not function if cards are put into your discard pile without being discarded, such as when you buy a card, when you gain a card directly (such as with Border Village), when your deck is put into your discard pile, such as with Chancellor from Dominion, or with Possession from Dominion: Alchemy, when trashed cards are returned to you at end of turn. It also does not function during Clean-up, when you normally discard all of your played and unplayed cards. The key thing to look for is a card actually telling you to "discard" cards, The Gold you gain comes from die Supply and is put into your discard pile: if there is no Gold left in the Supply, you do not gain one.