Garamsythe Waterway

"An underground waterway running deep beneath the Royal City of Rabanastre. Built and rebuilt over the centuries, no one alive knows all of its labyrinthine ways. Some are said to lead even to the royal palace. There are stairs leading down to the waterway in Lowtown, but they are normally barred to keep unwelcome visitors down in the sewers where they belong."

The Garamsythe Waterways are the sewer systems of Rabanastre. Within the waterways various dangerous creatures make their home, and would-be adventurers use them to sharpen their fighting skills. Unknown to palace security, there is a hidden passage from the Garamsythe Waterways into the royal palace basement.

It is accessed via traveling through Lowtown.

Monsters here:

* Dire Rat
* Steeling
* Ichthon (Docile)
* Lizard
* Malboro Overking
* Gespenst
* Ghost
* Garchimacera
* Water Elemental
* Baritine Croc
* Imperial Swordsman (Event Boss)
* Flan (Event Boss)
* Firemane (Boss)
* Cúchulainn (Esper)
* Gavial (Rare)
* Razorfin (Rare)
* Drowned (Rare)
* Wraith (Mark)
* White Mousse (Mark)
* Orthros (Mark)

Come here it you don't know the monsters.
http://finalfantasy.wikia.com/wiki/Garamsythe_Waterway