This tunnel runs between Route 5 and Route 6 and will allow you to reach Vermillion City. Because it's a bit dingy, travelers often lose their belongings here. Well, as they say, finders keepers! From the north entrance, there's a Repel nearby towards the right. Continue south a few paces, but slowly.
As you walk, pay attention to your partner's wagging tail. If you're observant, you should notice it wagging wildly a few paces south. Circle around here, massing the A button. With luck, you will find a hidden item. It's by the blue line on the floor, in-between the 3rd and 4th wall lights.
Hidden Items in the Underground
As far as we can tell, the hidden items you can find in this location are random, although they're typically Nuggets, Pearls, Potions, Repels and Lures. The cool thing is that the hidden items will re-appear the next day. So you can pick up another 5 daily.
Next, there should be another hidden item. This one is near the middle of the path, in line with the 5th pair of wall lights. Moving on, there's a third hidden item towards the left of the blue line, in-between the 6th and 7th wall lights. Plus a fourth hidden item by the red line near the same area.
After that is a visible Lure towards the left side, near the 8th wall light. Close by, search the red line next to the 9th wall light for the fifth and hidden item. That's everything in this tunnel. Once you're done scavenging around, exit the tunnel towards the south. Then leave the building to get some fresh air.