With Lumina defeated, there are no trainers left here that you can reach, so take a left along the North wall to go pick up a Hard Stone before returning to the front of the cave.  Take the center path this time and stick North to collect an Escape Rope.  This path joins with the left path, so continue West and then South and you will face another scientist in battle!  Beware, for Terrance’s Grimer uses not only Minimize and Mud Slap, but also Disable to make itself a right pain in the buttocks.

Battle: Scientist Terrance (864 Poke)
Opponent: Grimer Lv 18

Keep on to the Southwest and you’ll find a hiker who’s on what he calls ‘Wawalk’, or ‘The Waving Weaving Walk’- whatever that is.

Battle: Backpacker Kendall (432 Poke)
Opponent: Herdier Lv 18

There’s nothing you can do beyond Kendall, so it’s time to head back to where the cave broke into the sewers.  Take a quick diversion to where Burgh was lurking and you can pick up a Heart Scale from a nook in the West wall- and then you’re done in the sewers for now.  Use the Escape Rope if you’re worried, or just muddle your way out of the sewers yourself.

Iris runs over to you as soon as you reach the top layer of Thumb Pier, and expresses her concern- she thinks Hugh looked really upset when he came back out, and she’s a bit worried about what happened down there.  She asks about your encounter with Burgh, and suggests you should go challenge him- both to collect a badge and to improve his understanding of ‘you as a pokemon trainer’, whatever that is.

With pretty much everything you can do in this city done, patch up your pokemon at the Pokemon Center and roll out for the Gym, which awaits on the farthest-left street.  Before entering the gym, your team should be level 20 or so- level 19 isn’t too bad when entering, but if you’re mostly at or below that before even facing any gym trainers, you’re going to have a hard time in here.

Just within the gym doors, Clyde awaits, ready to hand you another bottle of Fresh Water.  You may want to take a moment to stand there and listen to the interesting variation of the gym music that’s playing here, but you’ll need to head through the cocoons and silken tunnels eventually.  Enter the cocoon in front of you and you’ll pop out on the next floor up.