2008 · ソースブック（プレイを拡張するルール／オプション）
Secrets of the Surface World
This sourcebook expands Hollow Earth Expedition with adventures set in the mysterious and perilous surface world, where criminals, mad scientists, dark sorcerers, secret societies, and world powers compete for control of the Hollow Earth. Characters can take the roles of daredevil pilots, private detectives, or wandering heroes as they explore exotic locations and disrupt dangerous schemes. It includes rules for psychic powers, sorcery, and weird science, along with expanded vehicles and equipment, new surface-world locations, and additional details on secret societies.
スキルベース（スキルを購入または獲得）ポイント制（ポイントを割り振ってスキルや能力を得る）ダイス（主にd2/コイン）
ゲーム情報（人数、プレイ時間、対象年齢、クレジット）の出典：BoardGameGeek。