From MHWiki

The Rodentia region was released on 4 June 2009 and consists of the open sea area lying to the east off the coast of Gnawnia, Valour, and Burroughs. Upon the initial release the S.S. Huntington II (see history) was the only location while two additional locations were added in August 2010, a fourth in August 2011, a fifth in May 2012, and a sixth on 2 June 2014. On 17 October 2017 the Slushy Shoreline was relocated to this region:

This fine vessel has been granted permission by the King to sail the open ocean east of Gnawnia, and has been upgraded with hundreds of thousands of interlocking hull fragments, allowing it to breach even the dangerous Iceberg with ease.
Crafting the S.S. Huntington III is needed to enter S.S. Huntington IV.
In recent days, huge chunks of ice have washed ashore the beaches of Gnawnia. Worse, dangerous mice have floated ashore alongside and are attacking the walls of Gnawnia!
Head to the beaches and stop the invasion!
You must be the rank of Lord/Lady to travel to this dangerous area.
Zugzwang's seasonal garden is the perfect destination for those who can't make up their mind about the weather. Every few days, the seasons shift, bringing with them a new crop of mice with different powers. Keep an eye out for the shards of an old key- they might provide the way into that mysterious locked tower.
Obtaining Zugzwang's Scarf is needed to enter Seasonal Garden.
An ordered chaos consumes this enchanted tower as the technological chess mice do battle with their mystical counterparts, all under the auspices of a dark game master who hides in the shadows. Conquer both Kings to find what lurks beyond the chess board, but beware: this is a place of powerful magics, where things are not always what they seem.
Crafting Zugzwang's Tower Key is needed to enter Zugzwang's Tower.
A magical barrier originally protected this library from mouse invasion until recently. Bands of magic and mechanical mice have circumvented the library's security measures in an effort to destroy the research Zugzwang once worked so hard to document.
Obtaining the Crystal Library Card is needed to enter the Crystal Library.
Iceberg, dead ahead!
Teeming with mice, and on a collision course with the beaches directly outside Gnawnia's castle, this Iceberg must be stopped at all costs!
You must be the rank of Lord/Lady to travel to this dangerous area.
Purchasing the Icebreaker License is needed to enter the Iceberg.
Dive into the depths of the Rodentia Ocean and explore this ancient underwater city! Great treasures await adventurous hunters who journey into the deep waters!
Purchasing the Sunken Souvenir is needed to enter the Sunken City.

To move to a new area within the game, click on the TRAVEL button. The map will show all your discovered areas, as well as any travel costs or special considerations of note in a specific area. It will also show the minimum rank required to hunt in that area.

(quick reference)
Gnawnia:  • Meadow   • Town of Gnawnia   • Windmill   • Harbour
 • Mountain
Valour:  • King's Arms   • Tournament Hall   • King's Gauntlet
Whisker Woods:  • Calm Clearing   • Great Gnarled Tree   • Lagoon
Burroughs:  • Laboratory   • Toxic Spill   • Town of Digby   • Mousoleum   • Bazaar
Furoma:  • Training Grounds   • Dojo   • Meditation Room   • Pinnacle Chamber
Bristle Woods:  • Catacombs   • Forbidden Grove   • Acolyte Realm
Tribal Isles:  • Cape Clawed   • Elub Shore   • Nerg Plains   • Derr Dunes
 • Jungle of Dread   • Dracano   • Balack's Cove
Varmint Valley:  • Claw Shot City   • Gnawnian Express Station   • Fort Rox
Queso Canyon:  • Queso River   • Prickly Plains   • Cantera Quarry   • Queso Geyser
Rodentia:  • S.S. Huntington IV   • Slushy Shoreline   • Seasonal Garden   • Zugzwang's Tower
 • Crystal Library   • Iceberg   • Sunken City
Sandtail Desert:  • Fiery Warpath   • Muridae Market   • Living Garden
 • Lost City   • Sand Dunes   • Twisted Garden   • Cursed City   • Sand Crypts
Hollow Heights:  • Fungal Cavern   • Labyrinth   • Zokor   • Moussu Picchu
Rift Plane:  • Gnawnia Rift   • Burroughs Rift   • Whisker Woods Rift   • Furoma Rift
 • Bristle Woods Rift • Valour Rift
King's Party Zone:  • Great Gnawnian Games
 • Great Winter Hunt
 • Halloween
 • Lunar New Year
 • MouseHunt Birthday
 • Spring Egg Hunt
 • Valentine's Day
 • Ronza: Ronza's Traveling Shoppe
 • Historic uses: Events
Others:  • King's Stockade   • Vacant Lot