Entitled The Land Beyond The Mountains, and published in 1982, shortly before the demise of Metagaming, that mega-campaign sandbox setting is comprised of two published adventure supplements, The Warrior-Lords of Darok and The Forest-Lords Of Dihad. Two additional adventure supplements were announced, for the provinces of Muipoco and Soukor, but never saw the light of day.
In this sandbox setting, all four provinces (Darok, Muipoco, Dihad and Soukor) are bounded by impassable hills, mountains and wastelands to the west, north and east. To the south is the sea. Each of the provinces are at odds with each other, and the overall setting is post-apocalyptic, with scattered ruins, mysteries, monsters and artifacts left behind by a vanished and technologically and magically advanced culture.
The adventure supplements for Darok and Dihad are slim: 32 pages each. An interesting innovation of this series is a fold-out 11x17 hex-map of the featured province, attached to the supplement cover. Other than the roads, rivers, settlements, mountains and sea hexes, the map is open for the gamemaster to add her own hex-crawl elements.
The first three pages of each supplement are identical. They briefly cover the history and legends of the overall mega-campaign region, and explain some of the shorthand and symbols employed throughout the supplements. The bulk of each supplement is filled with significant personalities, treatises on the culture of that province, along with sample towns, encounters and scenarios, and supported by several random tables.
The Land Beyond The Mountains is billed as a perfect vehicle for sandbox play: "Here lie rich deposits of gems and ore, and the buried relics of a forgotten golden age. Here are wizards and warriors aplenty, scheming to seize new territories for their liege lords or striving to keep the major trade roads safe and free for all. On these pages you will meet spies and scholars, raiders and traders, and many wondrous and dangerous beasts. In short, a myriad of opportunities for players to make and lose their fortunes, or attain positions of great political influence within a fast-changing, often unpredictable environment."
Even considering the thin-ness of the adventure supplements, I don't think the above statement is entirely over-reaching. After all, the benefit of supplement slimness is that it gives the gamemaster greater latitude and opportunities for sandbox play, beyond the adventure-as-written.