The Flatlands are the sections of land that surround spawn. They start out at 10,000 and usually end until 30,000-40,000. The Flatlands are usually generated with alpha chunks and beta chunks, but also have chunks generated by more recent versions.
It is very common to find abandoned bases in the Flatlands. While these ruins do not usually contain anything extremely valuable, they usually may contain items that could be useful to players who have escaped spawn. It is not advised to set up a long-term base in the Flatlands due to it being somewhat close to spawn.