The modern geography of Solus is largely determined by its tidal locking to the Sun. The surface of the planet is divided into two hemispheres. Heaven is bathed in perpetual sunlight, while Oblivion is shrouded in eternal darkness. However, Solus has not always been this way. Many millions of years ago, its rotational period had not yet been slowed to its orbital period by tidal forces, and the planet had a definite day-night cycle. Much of the surface of Solus was covered in a vast saltwater ocean, amounting to around 80% of the total area of the planet. However, as the rotational period of Solus relative to the Sun slowed and eventually stopped, the planet underwent a catastrophic climatic shift. As the dark side of the planet cooled in the darkness, the temperature eventually dropped low enough to freeze most of the ocean water of the hemisphere. Conversely, after being exposed to centuries of unceasing solar radiation, the seas of the sunward side of Solus gradually evaporated away. The rising hot air carried away moisture which was then blown toward the dark side by the winds flowing from the heat reservoir of the hot face. While some of the moisture precipitated on its lengthy journey and flowed back to the warm seas, some of this atmospheric water vapor was carried far enough into the dark hemisphere to condense and fall as snow onto the frozen seas below. Gradually, in this way, most of the water on Solus was sequestered in the gigantic frozen seas of what is now known as Oblivion. Where once there were oceans in the heated hemisphere, there now lie vast seabed valleys stretching for tens of thousands of kilometers in any direction. These seabed environments are rife with expanses of sand dunes and very concentrated salt flats.