Amidst the Overwatch Anniversary Event[1], Blizzard[2] revealed a new map for the hero shooter called Horizon Lunar Colony. As the name suggests, it takes place on the Moon. The company may have also hinted at a new hero called Hammond.
“Built as a first step towards humanity’s renewed exploration of space, the Horizon Lunar Colony’s goal was to examine the effects of prolonged extraterrestrial habitation—on human and ape alike. The scientists’ research proved incredibly promising…until, suddenly, all contact and communications with the base were lost," reads[3] Blizzard's description of the map.
Overwatch Anniversary Event: Every Skin and Emote Listed[4]
In terms of lore, it ties in with the origins of Winston - a genetically modified talking gorilla with genius level intelligence. Winston was one of the first characters revealed for the game. The Horizon Lunar Colony was where he was raised along with other apes.
Since it is on the Moon, certain sections of the map will have low gravity. Overwatch's custom game browser will let users apply low gravity throughout Horizon Lunar Colony if they so choose. The map is playable for those in Overwatch's Public Test Realm (PTR) and will be made available to the general public when Blizzard deems it fit.
Furthermore, the map seems to hint of a new hero called Hammond. The Overwatch Horizon Lunar Colony trailer explains its backstory as narrated by Winston, and a single frame shows all test subjects accounted for except for Winston and another known as Hammond. This ties in with a post[5] on Blizzard's blog that mentions Hammond being missing for a week. You can check out the preview for the map in the video below.