Pasture Land Sierra Island (島のシエラ) is a Man-made Floating Island located in Theia. It is the initial place where the Protagonist lives, along with Patty and Dorothy. Leon and Magna also lives here before.
Sierra Island is a small area, where the Protagonist initially starts his adventure. The only accessible parts are the upper area (where the viewpoint and their house is located), the lower area (where the railway terminal is located) and their house. there are also 3 Pigtapir marks in the area, and the place where one of the rare gifts, the Lily Bell, is located.
The island is not accessible until Chapter 2.
Sierra Island is where Leon and Magna lives peacefully, along with their son and daughter. Dorothy, her daughter Brigit, and sometimes, Erik, visits them occasionally until when Leon goes away to Marble Blue and disappeared and until Magna's Death. Dorothy takes care of Leon's son and daughter afterwards and takes responsible of them (in her older form) and also takes care of the Yakoos and Pigtapirs within the island.
After Mariv tells the Protagonist to go to New Paleo to start his journey to Marble Blue, he and Patty, left Sierra Island by riding the Monorail to New Paleo.
In Chapter 2, after the incident in Magma Ocean, The Protagonist goes back to Sierra Island due to Grief for the disappearance of Daisuke and Alfred, and feels responsible for it. After he saw Elroux in Sierra Island, standing in the top of the viewpoint, he was surprised that she was outside Mariv's Garden . Still on guilt, Elroux comforts him and tells the Protagonist that they we're still alive, just waiting to be saved, and asking him that she wants to see Marble Blue. With the newly found determination, thanks to Elroux, The Protagonist, Elroux, and Patty, left Sierra Island to meet Leary in New Paleo and start their rescue mission.