Do you want to know what it is, or where they are?
If you want to know what they are:
>Pokémon outbreaks were largely expanded in Generation IV. Once the player attains the National Pokédex, every day in a particular location, a "swarm of Pokémon" appears, which changes the probability of finding the given Pokémon from 0% to 40%. -Bulbapedia
But if you want to find a Magnemite, they are at Fuego Ironworks in Diamond and Pearl only.
To know when they are happening,
> Information about Pokémon outbreaks can be found in Sandgem Town by talking to Lucas/Dawn's sister or by watching the Sinnoh television DPPt, or by listening to Professor Oak's Pokémon Talk HGSS. -Bulbapedia
For more information, go here: http://bulbapedia.bulbagarden.net/wiki/Pokémon_outbreak
or click on any of the 3 links that I posted, they go to the same place :P