Avtually this thread has a lot more Tunnel discussion, if you're so inclined: https://www.boardgamegeek.com/thread/854026/playing-multiple-reaction-cards/
Summary: You discard x Tunnels, putting them all in discard, then reveal the first Tunnel from the discard and putting it back again, gaining a Gold on top of discard, then reveal the next Tunnel, etc. So you have to know which Tunnel (from the card position) you have already revealed. If you do it this (technically correct) way in practice, it's possible to cheat, because you're not required to show the cards you're discarding, just the top one (and the number), so you could try to reveal the same Tunnel several times, since it could be difficult for the other players to see exactly where in your discard pile you're pulling the card from. However, you're not required
to conceal all the cards you're discarding, so in the case of Tunnel, you could show how many Tunnels you're discarding as you're discarding them, then reveal that many from the discard pile. Of course in practice, this is done by just flashing all the Tunnels as you discard them and count that as also revealing them, so there is no problem.