Worlds in the Long Earth are named after the amount of steps, either west or east, that need to be taken to reach it from Datum Earth. For example, 'Earth West 5' is located five steps West of the Datum.
Many people left Datum Earth after Step Day, causing a decline in it's economy. One of the few boom elements was transport - Steppers can take you stepwise, but can't move you laterally at all. As a result, the Datum became the crossroads of the Long Earth.
This world's version of Yellowstone Supervolcano erupted in 2040. All the local versions of the volcano had been unstable, however this world was the only one that it happened on - scientists believed that this was because of the disturbances humanity had caused.
The term 'Datum Earth' comes from the definition of the word 'datum': "a standard position or level that measurements are taken from".