Line in the Sand is a new quest in Arc Raiders' Riven Tides update, though it's got nothing to do with the new seaside paradise. This actually continues the story of Stella Montis and Buried City, where Exodus materials were being transported.
You'll need to find four locations across Buried City to complete Line in the Sand, and they're not the easiest to pin down. I'll show you where you need to go below for the mission so it won't take you quite as long as it did for me.
Find out why the train never reached Marano Station
Your first stop is the blue derailed train cars between Warehouse and Piazza Roma near the centre of Buried City. They are also south of Marano Station following the tracks. Walk over to the train cars sticking out of the sand, and the quest will automatically update—you don't need to directly interact with anything.
Find out who orchestrated the robbery
Turn east from the derailed trains, and you'll see a small square tower up the hill, next to the wall perpendicular to Warehouse and Piazza Roma. Climb over the railing and interact with the detonator hidden by the bush in the southwest corner. This will spawn the Marano Market Business Card quest item, which is a clue for the next location.
Find out where the thruster valves were stashed
From the previous location, continue north past Library and Hospital to reach the Market Ruins area in the northeast corner of the map. Here you'll find a ruined building, which will progress the quest step—once again, you don't actually need to interact with anything here.
Search the basement of the merchant’s nearby apartment
From Market Ruins, head up the hill to the east, between two more buildings, to reach Dune's End. Enter the building and go through the door ahead of you, taking the zipline down to the basement. Inside you'll find a pile of cardboard boxes that you can photograph to complete the Line in the Sand quest.
