Saronia Catacombs is located underwater at the Saronia south border wall. Luckily, with your new airship Nautilus you can dive into water. 

Chests (13):

• Golem Staff
• Rune Bell
• Aegis Shield
• Reflect Mail
• Heavy Lance
• Elixir (x4)
• Phoenix Down (x4)

Steal (1):

• Gungnir (missable)

Follow the paths, collect all chests, until you open the hidden passageway. In there contains 8 trap chests; trap chests have monster(s) inside.

*** NOTE: Before facing Odin, change job one of your member to Thief to steal Gungnir (lance) from Odin. It is the strongest lance in the game with attack power 140.