From IvanWiki

The Underwater Tunnel is exactly what it sounds like: a long underwater tunnel connecting the island of New Attnam to the mainland where Attnam is. You must path through the Tunnels or over the water in order to fulfill the first quest of delivering the Encrypted Scroll to Petrus.

Entrance to Underwater Tunnels as seen in the World Map

Level Features

• UT1: Always has a "+" shaped room with stairs leading to the world map and a lantern.
• UT2: Always has two up staircases (to UT1 and UT3), and a large room with a diamond-shaped block in the middle. Inside the block are down stairs to UT4. Always very long horizontally.
• UT3: Always has a square room with a grass floor and up stairs to wilderness.
• UT4: Contains an altar to Silva. Many crystal growths can be found here of the 3 colors of light crystal, which are good light sources if you have a powerful enough pick to mine them.

Monsters

• UT3: Genetrix Vesana the Mother Carnivorous Plant dwells here. As long as she is alive, carnivorous plants of increasingly greater size and power will spawn in squares adjacent to you. Genetrix lives on top of the staircase leading out of UT3; therefore, she must be killed or teleported away in order to leave Underwater Tunnels.
• UT4: At least 3 golems, one of each crystal type, can be found here. Spiders and wild animals such as lions and bears are also common here.