From Halopedia, the Halo wiki
Fathom is a symmetrical map, and features themed coloring; one side is blue, while the other is red. The design choice was done to provide "that sporty, team feel". Certain areas of Fathom have long sight lines, though the sight lines change in other areas to create difficulties for long-range weapon users. The glowing buttons on the back of several ramps on the map can be shot, which causes the ramp to descend. As the back lane opens up, players with long-range weapons are given an advantage. However, these ramps only remain down for a few seconds before they rise back up and offer cover again. Cover in some of the hallways can be shot down to give players a full sight line. Small entrances allow players to slide through via Spartan abilities to allow for a quick escape from combat or to allow players to ambush opponents.
The Railgun is featured as the map's focal power weapon. The map also features LightRifles, Storm Rifles, SMGs, Boltshots, Battle Rifles and an active camouflage. The active camo powerup spawns on the central floor and the Railgun spawns on a platform above it—both an equal distance from both teams' spawning areas. If the player falls into the water surrounding the submarine, they will instantly die. Hallways toward the outer edges of Fathom create battle chokepoints as they allow access to the ramps that lead to the second floor of the central area, the location of the Railgun spawn. The open walkway area gives players a wide view of the map, but leaves them vulnerable to attacks from other players from a lot of angles.
Several sea creatures, like the Paelosur and the Qothal (known to the fanbase as "space whales"), can be seen swimming outside of the base; one of the Paelosur's is named "Timmy the Whale" by the staff of 343 Industries, named after creative lead Tim Longo.