Following your victory over Jasmine at the Olivine City gym, and once you've acquired the Mineral Badge from her, you'll be free to chase after the game's seventh badge in earnest. To do so, we're actually going to have to backtrack a bit, heading back to Ecruteak City. Now that you have the ability to Fly (which you earned in Cianwood City following your defeat of the gym leader there, Chuck), you can simply fly back to Ecruteak City instantaneously. Once you've arrived, head to the Pokemon Center there and heal up your squad, reworking your roster as you see fit. Then, when you're ready, head out of the city to the east, so that you find yourself on the yet-unexplored Route 42.

Route 42 is segmented into multiple parts, and you'll gain access to the various sections of Route 42 by heading through the gigantic Mt. Mortar dungeon. Indeed, other than TM65 (located just to the left of the entrance to Mt. Mortar), you won't find any other items on this westernmost segment of Route 42. And because of the lack of tall grass here, you won't even get to do battle with wild Pokemon, though that will be rectified once you finally reach the easternmost segment of Route 42. You will, however, have a chance encounter with a fellow Pokemon trainer as soon as you try to enter Mt. Mortar via the entrance nearby. After accidentally bumping into you and apologizing profusely, this man will hand over HM04, which will give you the ability to use the move Strength. Strength can be taught to multiple Pokemon in your possession, as long as they have the innate ability to learn the move. While Strength can be used within battle with great potency, its use outside of battle is what's really important, since you can move the large, rounded boulders you encounter out of your way.
You're going to need Strength in Mt. Mortar (and beyond), so be sure you have a Pokemon with you that can learn it, and teach it to that Pokemon before heading inside. Otherwise, retreat to Ecruteak City and take a Pokemon out of storage that will do the trick for you. Then, when you're ready, enter Mt. Mortar. The early-going is easy enough -- simply move northward, up the stairs, and through the door you encounter. From here, you have some options, but we recommend that you first head to the right, and go southward through the doorway you encounter. Once through this door, you can head south, and ultimately find yourself back outside, adjacent to Mahogany Town, which is our next destination. But there's no need to rush. Chances are, your Pokemon aren't quite leveled enough yet to face off with the gym leader there, and what's more, there is a whole lot left to see in Mt. Mortar, as far as items, trainers and wild Pokemon are concerned. So now that you know how to ultimately get to Mahogany Town, backtrack back inside the cave, and go north back through the door once more, so that you're on the horizontal pathway you were on earlier.

Now that you're back on the horizontal pathway, backtrack to the left. Pass the door you used on the far left to originally access this pathway, and walk up the staircases as they continue up and up and up. Eventually, you'll encounter a ladder leading downward. Take this ladder down to an isolated location, where an Ether can be acquired. Then, climb back up the ladder and backtrack all the way down the stairs. Run rightward down the path, past both already-explored doors. This should run you directly into Poke Maniac Harrison, the first trainer you'll fight here. Do battle with his Nidoking and Nidoqueen (both of level seventeen), and then climb up the stairs directly next to him. This series of staircases will lead to another ladder, this time heading upward to a separate isolated location. Walk down the stairs you encounter from here to grab a Revive, and then return to the ladder, climb back down it, and return to the location of Poke Maniac Harrison.In between the staircases you used to reach each of the isolated locations is another staircase. Walk up that one, and you'll encounter a stone in your way that must be moved with Strength. So, utilize Strength, and then walk past the location of the stone, heading in a northerly direction. Take the stairs you encounter on your left, and wrap around towards a dead end containing an Ultra Ball. Then, return to the previous corridor and continue to the north, and then to the east, grabbing the Max Potion at the dead end. Then, go north of that location and continue to the east. When you see a staircase leading to the south, take it, and continue to the south until you see a staircase leading back up to your left. This will ultimately lead to an isolated location, but you'll be able to take on Super Nerd Markus here. He has a lone level nineteen Slowpoke, so eliminate him quickly.

Once Super Nerd Markus is no more, there's little else to see or do here, though there are two more items we can currently get. Mt. Mortar is much, much larger than what we're able to see right now, but without certain skills, there's simply nothing else we can do here for the time being. To grab the last remaining items, backtrack towards the horizontal pathway at the north, and head to the right. Immediately to your south, you should be able to cop an Escape Rope. Then, if you climb the stairs above and to your right, and then head to the north and down another set of stairs, you should encounter a Nugget. Excellent! With both of those in hand, you are then free to leave Mt. Mortar for the time being. Remember how we just told you how to leave Mt. Mortar? Well, we told you that so that we didn't have to explain it again! So utilize that information we gave you earlier, backtracking and heading back outside to the eastern side of Route 42.
Once back out on Route 42, you can run almost immediately to the right to enter Mahogany Town. However, you don't want to do that just yet. Instead, you'll want to first fight the three trainers all around you (one of which you won't be able to avoid even if you wanted to). Poke Maniac Shane has two level seventeen Pokemon, a Nidorina and a Nidorino. Fisherman Tully, along the water to your left, has a lone level nineteen Qwilfish. And then there's Hiker Benjamin, with his level fifteen Diglett, level fifteen Geodude, and a level seventeen Dugtrio. So, as you can see, none of the Pokemon trainers have much for you to worry about in terms of their parties. As far as wild Pokemon on the eastern edge of Route 42 are concerned, well, you'll meet quite a few. Flaaffy, Spearow and Mareep are amongst the ranks of wild Pokemon here, so start capturing if you so desire. There are also water-based Pokemon to be found with both your Good Rod and Surf in the water to your left, including your typical Magikarp, as well as Goldeen and Seaking.

And speaking of that water, and specifically, speaking of surfing over it, you can surf to the left to find an isolated island smack-dab in the middle of Route 42. On this island, you can find a Super Potion. You will also find a cave entrance heading back into Mt. Mortar, and a sapling blocking your way southward. You can actually ignore the cave entrance all together, since there's nothing you can do in there right now (but worry not -- we'll return later). However, you should cut that sapling down with Cut (so be sure to have a Pokemon who knows Cut in your party!) When you do, you'll again run into that mysterious legendary Pokemon, as well as the man who seems to be endlessly chasing him, always a step behind. After that brief confrontation, you should pick the Apricorns from the trees where the Pokemon was hiding.

Thereafter, you can Surf back to the east, head eastward down the rest of Route 42, and finally enter the little hamlet there -- Mahogany Town. This place is small, so there's very little for you to see here. The southern edge of town is dominated by a gym on the left and a Pokemon Center on the right. We'll eventually get to do battle from within that gym, but to do so, we'll need to take care of some other business first (that's why there's a person blocking you from entering the building, after all). Do visit the Pokemon Center to heal your crew of Pokemon, however. The northern edge of town has some residential homes. The left one houses some sort of shady radio station. The two occupants won't be too forthcoming with information about what's happening there, but you can buy some items, if you so desire. Between the men inside the building and the older man walking outside, there's a lot of buzz about Gyarados around here. Just what's going on?