Access conditions: The door must be covered in precipitation. Rain, mist or fog, hail — anything goes. Furthermore, it is impossible to open the door from outside if there are visitors inside. If a door is opened, it leads to latest access point ro re-emerge from water.
Size and Shape: Spherical, infinite but cavern system stretches up to ~25km from the center.
Biome: Caverns. Half the cavern system is filled with water at any given time.
Peculiarities: The cavern system is continuously rotating around its longitudinal axis, with a period of rotation equal to 25 hours. Six entry points are strewn around the caverns; as such, different points are accessible during different times of day.
Flora and fauna: The walls of caverns are covered with edible bioluminescent moss, which produces oxygen to breathe. The fauna is composed of hostile, cannibalistic omnivorous swarms of piranha-like fish.
Sapient activity: None.
Access conditions: The door must have a maze carving on it.
Size and Shape: Roughly conical. Actual size imperceptible.
Biome: Caverns, arranged into a maze-like configuration. An undefined amount of "upper-level" caves host entry points, unique to each entrant. The arrangement of the caverns suggests that the explorers are meant to descend down; most of such descents do not require any spelunking skills and may as well be done by wheelchair, if the situation calls for it.
Peculiarities: As the explorers advance, they might need to press buttons which will seal off some passages and open up another. Furthermore, those who stay for more than six hours start developing a kind of addiction to the realm which makes creates mental compulsions to return if they leave and continue descending for as long as they stay; this addiction will additionally manufacture various excuses to do so based on out-of-realm justifications. This addiction is only curable by making the explorer unable to access the realm for five times as long as they've been exposed to it.
Flora and fauna: None.
Sapient activity: None other than that of the explorers.