Sandbox

Sandbox is upcoming map that will be included in the Mythic Map Pack. Its setting and architechture is similar to Sandtrap, but its forge capabilities seem to be on par with Foundry. The level's hidden skull logo, a Monitor, supports the theory that the map will be Forge based. In certain Bungie.net forum posts, there have been repeated queries as to whether there will be a larger version of Foundry, and in some cases, Bungie staff or higher ranked members have replied and said that there was one in the works.

Sandbox along with Assembly and Orbital have been confirmed to be shipped with the Limited Edition of Halo Wars. Bungie has confirmed they are working to get these maps available on Marketplace as soon as possible after Halo Wars' release on February 27, 2009. The release date for the other three maps is unknown although it is speculated that they will come with Halo 3:ODST.

"Forerunner structures are often so vast, so towering and so exaggerated that they'd be visible from a number of different excavation sites."

- Lukems posted a brief clue about Sandbox's location in the Halo universe on 01.20.2009 2:40 PM PST

This may refer to Sandtrap as it was a desert-based excavation site. It may also refer to the tower of Epitaph as it is very large; and the tower in Epitaph resembles the tower depicted in the sketch of Sandbox, but it could also be one of the Sandtrap towers.

Forge
The center of the map is a flat, empty floor, similar to Foundry but visually resembling Sandtrap. The map is built from forge-editable blocks, pillars and ramps. The player can also place halves of the pyramid structures on Sandtrap. Sandbox will likely have more vehicles available the Foundry however the confirmed vehicles are Warthogs, Mongooses, and two Choppers (The second is slightly visible in the wide-screen picture). A new Forge item is an orb of light that colors its surroundings in either red or blue; it can be used to differentiate the two bases, similar to most maps made by Bungie. The floor may also be removeable to access the blue grid shown in the video of their Interactive Innovation acceptance speech at the end.

Skull
Sandbox features a hidden skull, the Sandbox Skull. This skull, if obtained, unlocks the Sandbox Skull Achievement. The crest of the hidden skull on this level appears to be a likeness of the Monitor.

Trivia

 * In the new screenshot released by Bungie, Sandbox seems to be a small-medium sized map, possibly about the size of Standoff, with obvious resemblence to Sandtrap. Sandbox may be the desert-theme map of Foundry as the structures within the screenshot are speculated to be scenery objects players can move. As stated by Bungie, Sandbox combines large-scale vehicular combat with a limitless amount of Forge customization. Boasting a tiered, open-world layout, “Halo 3” players will be able to build their own unique game experiences using the Forge map editor. Sandbox is a map unlike any Bungie has ever created.
 * The vehicles on map, which are the Mongoose, Warthog and Chopper, are the same as Sandtrap, with the only difference having 2 Warthogs less, possibly the respawn times for the vehicles are longer and the land surfaces are more even.