This tunnel connects Route 8 and Route 9 and is the main way of getting to Celadon City, at least until Saffron City unlocks. Similar to the one from earlier, go down the staircase to the left when you enter. In the main tunnel area, slowly proceed west while counting the lamps on the wall above. Yup, it’s time to go item hunting again!
When you’re halfway between the 2nd and 3rd lamp, search near the red line for a hidden item. Next, there’s another hidden item if you search the blue line halfway between the 4th and 5th lamps. The next item can be found just under the 6th lamp: it’s an X Accuracy that’s lying in plain sight.
Moving on, carry on until you’re halfway between lamps 7 and 8. Search the blue line for your third hidden item. Then go towards the halfway point of lamps 9 and 10. The fourth hidden item can be found near the middle of the path. Finally, there’s an X Defence south from lamp number 11. Further west are the stairs leading back up, where the exit is located.