Underwater City

Underwater City is the First Sea island for levels 375 to 450, and the way in is a whirlpool. Its entrance sits between Frozen Village and Prison, surrounded by three large rocks, and because the island is small it can be hard to spot. A dome keeps the water out, a large castle stands in the middle, and the outer edge is lined with tall rocks and sandy ground dotted with mushroom-like trees. King Neptune runs all three quests: 8 Fishman Warriors at Lv. 375 (8,750 Beli and 3,050,000 XP), 7 Fishman Commandos at Lv. 400 (9,000 Beli and 3,350,000 XP), and the Fishman Lord boss at Lv. 425 (15,000 Beli and 4,250,000 XP). The Water Kung Fu Teacher also lives here, which makes the island the place to learn that fighting style. Two secrets are worth a look: clear bubbles carry you up to the Coral Cave and its chest, and a crystal puzzle to the right of the Fishman Lord's castle frees the Water Kung Fu Teacher. Do the quests in order, then leave for Upper Skylands.