Ocean Gate

Ocean Gates are magical portals that are located deep in underwater of each planet in Magical Dimension. Each Gate is guarded by Selkie

Appearance
Gates appears as two really sharp rocks, that almost forms an circle, leaving an empty space between their upper spikes. portal between these rocks appears to be invisible and it needs Selkies magic to go through them When something gos through the Gates or Gate is activated, it glows brightly. Apparently, only sea-creatures needs these Gates, because land creatures prefers to teleportation magic and spaceships, though using Gate is seemingly faster.

Known Gates
Gate in Andros. Its located very near the deep Prison of Andros and is guarded by Lemmy

Gate of Earth. Its located near Gardenia and is guarded by Fella.

Gate of Solaria. its located in Royal Deeps and is guarded by Aliris.

Gate of Melody. Its located in Ruby Reef and guarded by Sonna.

Gate of Magix. Its location is unknown, though its implied to be in the lake near Alfea. Its guarded by Nissa.

Season 5
At Gardenia in planet Earth, an oil spill occurs and oil floats its way deep into the ocean, eventually reseaching the Gate of Earth, which is guarded by Fella and one of Andros triton guards. Fella tries to stop oil with her magic, but oil appears to be immune to her powers and so it enters the Gate to Andros, where Andros gatekeeper Lemmy and Fellatriesto stop it again. After stealing Lemmys and Fellas powers, mutated Tritannus decides to seek all other gatekeepers in order to enter Infinite Ocean and rule Magic Dimension. To do this, Tritannus uses Gates to reach other worlds and their gatekeeper Selkiessohe can steal their powers and control their Gates. So far Tritannus uses Earth gate the most, in case he runs out of toxic that grants him his powers. He would send Trixsaheadto dumb toxic into water or sabotage oil plants. He himself would use the gate to go absord this toxic. Winxs havent used any of the gates yet as they prefer using magic.

Trivia

 * Seemingly, every world has one Gate, except Andros, which seemingly has counterparts to all other Gates.
 * It is unclear if Infinite Ocean has gate or not. It may have, but it may also require powers of many Selkies before it is possible to enter.