The island of Mossdeep is covered in plants and flowers. Its weather is mainly sunny, and its winds are stable. This clear weather made Mossdeep the ideal location to build a space center to launch large rockets. In Mossdeep, there is a white rock which holds a special meaning to the people at the space center. They placed the rock outside of the space center as a symbol of their wish for their rockets to fly safely. After entering the Hall of Fame, the player can obtain a level 5RSE/1ORASBeldum from Steven's house.
Mossdeep was the ideal location to build a space center which launches huge rockets because of the stable weather conditions. The Space Center is successfully sending rockets into space. In Emerald, the Space Center plays a big part in the plot, where Team Magma attempts to steal rocket fuel while a rocket begins to launch into space. On the second floor, the player will meet with Steven and battle two Team Magma Grunts.
Visit the house nearest the stairway to the Space Center to meet a young woman who uses a Wingull to send letters to a friend in Fortree City. Talk to the Wingull and it will take flight, off to deliver another note. If the player return to Fortree and visit the Pokémon Breeder in the northeast house there, it will get a Mental Herb that his Wingull found in its travels.
In Pokémon Ruby and Sapphire, the population of Mossdeep City is 45. Despite it being on an island in the middle of the sea, its population is fairly large, and it is Hoenn's fourth most populous city.
In Pokémon Emerald, the population of Mossdeep City is 52, making it Hoenn's fourth most populous city.
According to the anime, Mossdeep City is located on Mossdeep Island. As mentioned in Judgment Day! and Clamperl of Wisdom, ferry services run between Mossdeep and some of its neighboring islands, as well as Lilycove City.
In a flashback in PS600, Steven invited Ruby and Emerald to visit his house in Mossdeep City to tell them about the giant meteoroid that was threatening the planet and giving each of them a Mega Bracelet to help stop the meteoroid and anyone who would try to sabotage the Devon Corporation's efforts in stopping it. Being skeptical of this information, Ruby accepted Steven's gift only on the condition that he'd not tell Sapphire about the meteoroid just yet.
In another flashback in PS602, Ruby and Emerald sat by the sea at Mossdeep City while pondering who they could turn to for advice in order to stop the meteoroid without making people miserable. Ruby figured that Courtney could be such a person, and used a recording of her memories to look into her past, learning that Zinnia had been the one to tell Team Magma everything about Groudon.
In PS605, a rocket was seen awaiting launch at the Mossdeep Space Center, its purpose being to create a warp hole to transport the meteoroid away. Sapphire, having been transported to the Space Center by Hoopa, gave Professor Cozmo the rocket's final and most important component, the dimensional shifter. When the information about the meteoroid was leaked to the public, masses of people gathered around the Space Center for answers. In PS608, the rocket was successfully launched, but Zinnia, bearing a searing hatred for the Devon Corporation, attacked the rocket mid-flight. Despite Sapphire fighting against her, she was able to make the rocket crash and destroy the dimensional shifter.