The mysterious world of the Nessies is called the Melange by human visitors due to being a surreal blend of both familiar and completely otherworldly entities/phenomena; with some biomes where the inhabitants truly (and correctly) looks like they came from different worlds.

The Melange takes place on a planet that is geologically weakened; it’s tectonic plates stopped moving a millions of years ago, with vast cave systems that lasted very long due to the lack of earthquakes. While the Melange planet/universe have physical/geological laws as we know it, it also has its own metaphysical quirks, with the most obvious example being crescent gems that produce electromagnetic radiation like some sort of magical crystal stars.

The Nessies categorize their world in 3 major layers: MiddleNet, DeepGulfs, and OuterLands.

The OuterLands is basically the surface of the Melange Planet. The effects of a lack of plate tectonics are visible on the landscape: due to vagaries of winds and waves, there aren’t that many mountains and the remaining mountains are short and smooth. Since the plates above any hotspots doesn’t shift, any volcanos can grow to enormous proportions as lava keeps piling up. The surface is also dotted with jagged craters as meteorites occasionally smashes into the planet. The biomes here are mostly composed of Exo-Forms, extraterrestrial creatures that might or might not originate on the planet, and Terran (Earth-derived) life forms are uncommon here, implying that environmental conditions might be very different on the OuterLands compared to Earth’s surface...
The MiddleNet, filling most of the Melange’s lithosphere, are the cave networks which are the most hospitable to Nessies, humans and other Terran life forms. In addition to millions years of water based erosion, many of the caves have been shaped by burrowing life forms as well. Despite being subterranean the MiddleNet have several familiar biomes; thanks to the crescent gems and their seemingly endless shining, photosensitive organism like plants and animals with eyes thrives there. Since crescent gems are unevenly distributed across the Melange and can vary in what wavelenghts they can produce, the caverns are quite diverse including dark places with gems that shine very dimly and/or produces longer wavelenghts (like infrared) while other areas are as bright as a summer day (with some of these even requiring some sunscreen for extended stays). Most of the inhabitants here are Terran, although their evolutionary lineages diverged long before modern mammals and even dinosaurs evolved, but Exo-Forms dwells here as well, with Exo-Forms populations being more common and diverse in MiddleNet areas closer to the surface.
The DeepGulfs are the inner parts of the lithosphere closer to the mantle. Magma chambers/tubes are more abundant there, especially active ones. While precious resources and even some extremophile ecosystems can exist there, the Nessies prefer to keep a safe distance from these boiling hot places. Especially when there are many accounts of catching glimpses of humongous, mysterious entities that seems to dwell in the most infernal depths of the DeepGulfs.