Explorer Echidna is an unreleased episode, featuring its protagonist Knuckles The Echidna, who is exploring the Mystic Cave.
The story starts with Sonic playing a game on mobile phone which is techically Sony Ericsson Xperia (Knux95's old phone). Knuckles, in the mystic cave and on his mine cart, is trying to find Rubys and Diamonds, and after this, he will find a Golden Ring statuette.
Extended information will be found later when this video is released.
(The information and storyline may not be officially used in the main release)