The Capital of Rain is a planet with never-ending rain.
According to Maetel, the Capital of Rain has been under a downpour since it's creation as a planet. One side effect of it's climate is highly accelerated plant growth that allows a tree to become fully grown in only a day. Another side effect is that the constant cloud coverage blocks the sun from being visible, meaning that nobody on the planet has ever seen the sun. To prevent constant flooding, the ecosystem naturally developed in a way that created a sponge-like material beneath the surface that soaks up excess rain water.
The wealthy citizens of the planet are allowed to live on the surface of the planet. Those who are less fortunate are made to live over 600 feet beneath vast pools of mud in one-room bubble houses. Despite being beneath the mud, these bubble houses are able to receive television and radio transmissions to do the society's vast media transmission technologies. Despite the segregation, the poor people on this planet have much easier lives than those on other planets in the Galaxy Express 999 series.