Update: Splatoon and Mario Kart 8 for Wii U taken offline.
Update : Nintendo has announced that it has begun temporary emergency maintenance on Splatoon and Mario Kart 8 for the Wii U.
While unconfirmed, it's heavily speculated that the maintenance - which at the time of writing has no time frame attached to it - is linked to the 'ENLBufferPwn' exploit detailed in the article below.
Read the full article on nintendolife.com