Sootopolis City is located in the crater inside a dormant volcano. As such, this city requires you to Dive to get into it. In the middle of the city there is a massive pool of water, with the residents living in buildings on the sides. It gives access to the special Cave of Origin and the final battle between Groudon, Kyogre and Rayquaza occurs in this city.

e-House
In Pokémon Emerald, there is a house with a backroom blocked by boxes. This house opens up when you scan in various trainers using e-Cards on the e-Reader. These trainers are then battleable constantly from there.

Move Tutor
In Emerald, in the Pokémon Centre, near the bookshelf, there is a trainer who will teach your Pokémon the move Double-edge. This is free but can only be taught once

Name	Type	Cat.	PP	Att.	Acc.	Effect
Double-edge			15	120	100	A reckless, life-risking tackle. It also damages the user by a fairly large amount, however.

Gym
The Sootopolis Gym is the final gym of the region and features a puzzle where you need to move around an icy platform, making each tile crack before getting to the end. If you step on the same tile twice, you will fall into the bottom floor. Get to the end and you will face the Gym Leader; Wallace in Ruby/Sapphire or Juan in Emerald. Defeat them and you'll earn the Rain Bridge and TM03, Water Pulse

Special Moves used in Sootopolis City:
Surf
Dive