Atropus

Atropus worlds are living and dead, all at the same time. Experts disagree on their nature, some saying they are experiments while others insist that they are literal breathing planets. Flesh is the number one export of these worlds, followed by lovely and decorative bone. While they do have rough weather from time to time, your biggest worry here is usually being really grossed out.

Notable features
Upon arrival, the first thing the explorer will notice is that the surface of Atropus planets are literally Flesh and Bone. Besides flesh, bone and large pools of bodily fluids (namely pus, blood, or alien juice), travelers can find the usual assortment of valuable ores and minerals, the veins growing richer the further down one mines (though mining in Atropus worlds is more of a flesh-cutting affair than a rock-crushing one).

As to the origin of these strange, maddening worlds, the experts haven't arrived at a consensus. Some believe that Atropus is simply the natural culmination of life's evolution: given enough time, the biosphere of a planet will eventually evolve from a multitude of interdependent living beings to a single, planet-wide living organism. Others think that they are the results of advanced genetic engineering from some long-lost alien race, a lonely mad genius, or a long-lost alien race of mad geniuses. Yet others uphold more esoteric theories involving arcane-sounding things such as "Great Old Ones", "Primordial Evil", "Forgotten Lore", and "Embodiment of Madness"; a few of them even combine all of these weird concepts together!

The core of the planet is either Pus or Blood instead of Lava.

Upon beaming down, you will start gaining Madness currency over time. This is accompanied by debuffs that lower defense/energy over time, etc. If you stay in this world for a long time, your defense and energy can reach into the negatives, making damage potentially fatal. Also, some creatures (including Kevin) will be hacking into your SAIL console and sending demotivating messages including death threats, periodically. (All the effects can be negated if you have Insanity immunity, e.g. from Darklight Flower).

Trivia

 * Atropus worlds are based on the Forgotten Realms world of the same name.