Relic Passage (South Entrance)
In here, we find the Relic Passage, or at least, on of the entrances to it. Immediately going left is an Escape Rope if you don't feel like walking out of the sewers to get out. Going to the far right, up past a scientist, and then left some is a Hard Stone, and item that boosts the power of Rock-Type moves. The scientists in here use X Items to power up their Pokémon, which gives you a chance to attack. However, there aren't many trainers that are accessible from this entry way, so there is not much to fear.
Also in this cave is some new Pokémon. First is Woobat, which is probably the first thing you will encounter in this area. Where Zubat can evolve into Crobat, where it has good speed and attack stats (and even Sp. Atk, if you want to go 2010 World Championship Crobat), Woobat evolves into Swoobat, in which it doesn't have too great of offense or defense, only speed. Its abilities are useless, in which the only useful one is its Hidden Ability, but you don't get access to that until the after game. Next is Roggenrola, the "Geodude" of the game. If you have someone to trade with, then Gigalith is a pretty big threat, boasting a 135 Attack Stat, and a 130 Defense stat. However, if you can't trade, then you are stuck with Boldore, which is not bad by any means (especially with Eviolite being a thing), but it does not harness its true potential. Lastly is Timburr, which is in a similar boat as Roggenrola. It is the "Machop" of the game, as you need to trade to get it fully evolved. It has good Attack and defensive stats when evolved, but it misses out on a lot of the moves that you would get from breeding like Mach Punch and Drain Punch, but it is still a good Pokémon, none the less. It even learns Rock Slide and Stone Edge level up!
Since there is not too much to see, there is not much point to staying, so returning to Castelia would be a good idea now, since you have not been healed for a while now, and you have a bit of a backtrack to a Pokémon center.