From Halopedia, the Halo wiki
Waterfall was a multiplayer map developed for and intended to appear in the now-cancelled Halo Online. In Season 6: Raven, the map was added to the Halo 3 component of Halo: The Master Chief Collection, the first map to be added to the game since the Mythic II Map Pack in 2009.
Universe and lore
On a far-flung world in the Zezar system; Zezar VI, the UNSC has maintained a network of facilities acting as a fallback option for key personnel and assets. In addition to providing shelter from hostile encroachers, the UNSC has taken great interest in what might lie beneath the planet’s icy surface, as well as peculiar findings on its two prominent moons.
Waterfall is one of several levels intended for inclusion in Halo Online, prior to the game's cancellation. Prior to 2021, little was known about the map as it was never featured in any of the Halo Online testing releases; the only information relating to the map was the text strings in the MS30 Halo Online build referencing the map's name and description. Some concept art was posted online by Pavel Belov showcasing the map's described setting of a hidden UNSC base.  In February 2021, it was announced that the Season 6 Flight build for The Master Chief Collection would contain the map Waterfall for the game's Halo 3 component; following the release of several armor permutations cut from Halo Online in the prior season. This marks the first time Halo 3 has been updated with new developer-made (non-Forge) maps since the release of the Mythic Map Packs in 2009 and the first Halo Online map to see renewed life outside of the original game.