I imagine most of those who frequent this forum already know about this combo, but the specifics are kind of interesting.
First, let's talk about these cards separately. As alluded to in another thread, Cartographer can be a bit of a trap card, as it's easy to fall in love with its sifting power only to realize that you haven't bought enough good cards to actually sift for. In addition, it's easy to trigger unwanted reshuffles, and since you've sifted through all your junk, your next few hands will be mostly that junk.
Tunnel can be another trap card. At first glance it either looks so powerful (2 VP for $3 AND you can get bunches of Gold?) or so weak (only one VP more than Estate, and how am I ever going to trigger the Gold-gaining effect?). Then you realize that there are tons of cards that cause discards and maybe you buy a Tunnel, only to find that either you can't trigger them as often as you'd hoped (Oracle, opponent's single Militia), so you buy more Tunnels, and now you have tons of Gold, but your hands are all Gold-Gold-Tunnel-Tunnel-Copper. Sad panda.
Putting these cards together helps them solve each other's weaknesses. Cartographer's 4-card discard reach and the ability to chain multiple Cartographers means you don't need very many deck-clogging Tunnels to reliably trigger the effect. The Gold-gaining effect solves the dilemma of having to spend your buy on Cartographers and still get good cards to sift for. Add it up and you have a combo that's reasonably competitive.
However, it's important to make note of two things. One, while this combo is reasonably good, it's inferior to both Smithy-BM and Envoy-BM. (It has a narrow margin of victory in the simulator, but that's because the default bots don't buy Tunnel. Add a buy rule to buy Tunnel at <= 3 Provinces and both Smithy-BM and Envoy-BM pull ahead.) Two, it's a huge dog to at least three other Tunnel strategies: Young Witch/Tunnel, which is a beast, Warehouse/Tunnel, which gets going fast, and Embassy/Tunnel; this is because while Cartographer reaches 4 cards into your deck, it can't discard anything in your hand, so having both in the same hand gives you the double whammy of having a dead card *and* not being able to find/discard it for Gold. It's pretty good when there's not much else going on on the board, but it's not always going to be the dominant strategy, and it's not that fast (it derives most of its power from the additional VP from Tunnels, which you can hit early without clogging the deck).
Also of interest is that this strategy plays well with other cards, unless they're terminal draw. Cartographer/Monument/Tunnel is nearly a 3:1 winner over Monument-BM, and Cartographer/Mountebank/Tunnel is a 60-40 favorite as well. Cartographer/Witch/Tunnel, on the other hand, is nearly dead even with just plain Witch.