The tunnel graves

There's a reason humans came out of the Dark Below: here lays things that can and will kill you in so much density you probably are safer taking a nap in the middle of a sieged city .

Geography

South of the island lays a numerous amount of tunnel formations that connects with cavernous holes below it's earth. The underground contains small patches of underground lakes, but the most impressive aspect is the tunnels which seems like an oversized ant hill seen from the inside. The place is constantly active and numerous sounds can be heard as the 'Molder Worms' frequently come and go in the tunnels, creating even more passage, creating a ever-changing underground labyrinth which changes over time.

Ecosystem

Three species are the main actors of the ecosystem.

• The Molder worms: gargantuan-sized toothy worms, they regularly aerate the soil with their tunnel formations. They usually are so big nothing refrains them from their path when they travel underground. They always are on the run to find more nutrients. Their network of tunnels is so immense that it connects far beyond the island, connecting to other ones around the archipelago.One could use this network to travel from an island to another, but survival is extremely hard if one worm happens to use the tunnel at the same moment. They also dig deep as they can eat patches of minerals of long sustenance.
• The Carcolians : While not necessarily evil or ravenous, those sapient snail creatures can cause hazard by themselves to the unwary traveler: they produces various substances from their body as they travel around, most having a strong narcotic effect which they are immune. Touching that slime unprotected can severely impedes a underworld traveler's health if left unchecked, leaving it very vulnerable to other preys. The carcolians like about any type of ecosystem below, but they recently try to expand their territories above the earth level. While they are slowly, they are steady and smart.
• The Wehiika : The most dangerous local species of the island's underground. A mutated species that received cursed blood transfusions from the Serpentfolk since ages immemorials. While those were planned to act as a new striving but controlled race for their makers, the cataclysm left them devoid of source of knowledge and power ever since. Since then, they resorted to tribalism and following of old scriptures, trying to bring back their cursed pantheon and masters of old. They usually work outside the underground depths, prefering to stay nearby large bodies of water, and kidnapping preys to sacrifice and consume. They control a majority of the underwater biomes below the earth.

Localized Phenomena

Because of the frequent remodeling of the tunnels by the molder worms, the place is at times at risk of tunnels collapsing on themselves if the connections makes it happens. this makes the traveling even more dangerous and unpredictable. The carcolians can usually avoid nearby collapse because of their slimes which the worms dislikes, enforcing them to dig elsewhere, and the Wehiika  near the surface sees this occurence less often because of the favorable depth position.

Natural Resources

While they place is not scarce of minerals, most of the molder worms take a jump at the biggest formations, which makes mining and spelunking less profitable as only small ores remains available to mine.