The mountains of the Mountain Sector are shrouded in fog, there is more fog covering the Digital Sea. The sector is filled with moving pieces and narrow pathways. Due to this it can be hard to find towers. In Season 1, the mountains were a different color, being a dull and dark shade of purple. Normally when towers are activated in this territory, Bloks and Hornets guard them because they have the advantage in this territory. Appearances by the Scyphozoa and other monsters have happened as well. Sometimes, when the Lyoko Warriors jump off a platform, they are found in a different part of the sector. This sector was destroyed by Aelita by entering the Code, X.A.N.A. in the way tower in Double Trouble, being the last of the first 4 sectors to survive.
The mountain sector survived between seasons 4 and 5.
This sector never had a replika shown within the show. It's probable that there was one, as when the Kolossus was first formed, it was shown that there were dozens of replikas.
XANA can control the digital sea, at least in this sector, as he was able to flood it in the episode, Tidal Wave. This may either be an aspect of the Sector Alteration Program or Lyoko's floodgate.