The Hosts took control on a day known simply as the "Arrival". That day began with massive worldwide communication interference and jamming, which came after a weeklong hunt for relevant key figures around the world. Late that day, massive rectangular blocks descended from the sky, linking together to build walls dividing the city. One of these walls, 20 to 30 stories tall, many meters thick and many miles in length, surrounds the central part of Los Angeles, where the series is set. Other similar walls have been constructed around neighbouring urban areas, called "blocs", with the whole referred to as a "colony". Traffic passes through the walls at heavily secured checkpoints, called "gateways", which allow the Authority to strictly control the movement of people and the distribution of consumables, such as food and fuel, which are rationed. The geographical extent of the alien invasion is unclear, but later scenes in the series shows Authority members from all over the world - hence making the invasion scale worldwide.