The Mountain Island is a large island biome. It is located on top of a mountain range in the Mountains. It is located north of Lifepod 5 and is next to the Bulb Zone, Kelp Forest, Mushroom Forest and the Underwater Islands. The Mountain Island also harbors the Quarantine Enforcement Platform at its northern tip.
The Island is shrouded by fog, so it isn't visible until one is relatively close to it. However, the player can receive coordinates for its location from the Sunbeam.
The Mountain Island is characterized by rocks of various sizes and a fairly large mountain in the center, surrounded by an extensive shoreline.
The Island is also connected to the Mountains Caves, with its entrances accessible from both underwater and on land.
The Mountain Island can be fully explored on foot, featuring a small variety of flora, like Bulbo Trees, bioluminescent grass and Pink Caps as well as Speckled Rattlers. This island, unlike the Floating Island, is part of a continental shelf. The path to the summit is marked by Precursor technology; along the path, two Purple Alien Tablets can be found. Near the summit of the island is a Principal Alien Arch Cache connected to the Floating Island Arch.
- The caves leading to the Alien Arch are extremely rich in rare resources whose acquisition normally requires exploring much more dangerous biomes. Shale Outcrops in particular are abundant, providing easy access to Gold, Lithium, and Diamonds for anyone capable of handling a large number of Cave Crawlers in the area.
- If the player opens their PDA in the direction of the Mountain Island from anywhere on the map, like a spyglass, the clouds shrouding the island will glow a bright white. This shows the player in which clouds are real or fake, which reveals the exact location of the island. This method of finding the island works regardless of which PDA menu the player is on, but it only works during the daytime and while above the surface. It also works on the clouds shrouding the Floating Island.