Living underground on such a planet can be challenging, since the solid ground needs to be strong enough to hold together instead of getting blown up and launched into space by the reversed gravity. Most creatures might need to utilize nature cracks and holes in rocks, or be less aggressive than earth animals in digging tunnels. We can also assume that there are large cavities underground filled with liquid or gas, forming ecosystems. Some possible creatures are as follows:

- Shallow diggers

These creatures are between fully underground and the *Ground Dwellers*. They slowly dig holes on the surface of the planet, then hide and stick themselves inside to avoid getting ejected out by "gravity". They might look like some mollusks in our Earth, with a smooth, thin and long shape. Some of them might live on filter feeding, while others use tentacles to capture smaller creatures in the liquid layer, or even just get a biological "safety rope" and then release the hold, letting gravity shoot them out towards the prey.

- Crack crawlers

They are small or thin creatures with mechanisms to grip onto the stone walls and move through the cracks under the ground. Some of them would be amphibious to adapt to the liquid seeping in the cracks from time to time. With almost no light in their lives, except for those who would occasionally return to the surface, many of them would lost their eyes and get a great sense of touch instead. 

- Hot spring inhabitants

They would be the foundation of the "cavity ecosystems". Without light for photosynthesis, they turn to chemosynthesis using inorganic chemicals from hot springs in the cavity. These creatures might be bacterias or look like giant tube worms just like their relatives on Earth, but the shape could also vary alot due to the strange gravity. They might even manage to help produce some soil-like material by breaking down the stone wall and adding organisms to it, leading to a quite interesting effect: while a liquid lake stays at the bottom of the cavity, the soil is stuck to the "ceiling" on the other side by reversed gravity.

- Cavity walkers/flyers

The first are walking animals living in cavities, either holding onto the bottom or standing on the ceiling. They might mainly live on the ceiling to save energy, but could grip onto the wall and climb to the other side for water or other resources. There is no need for eyes, but they would have superior sense of touch, hearing and smell. If this world could allow large cavities to form, where there is enough energy to support flying creatures and enough space that flying can be an advantage, we might get flyers. They could use echolocation to navigate in the darkness and complex wings with multiple joints for better efficiency.
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