The Gears 5 Hollow Storm side mission is one that you can complete in Act 2, when you reach the open world snowy area. There's three locations to visit in order to complete Gears 5 Hollow Storm, and we've got the whereabouts of all three including how to complete each of them in Gears 5. Of course, if you haven't reached this point in the game yet, stop reading this Gears 5 Hollow Storm guide now in case of spoilers!
Gears 5 Hollow Storm: Train Tunnel
You can start the Gears 5 Hollow Storm mission by visiting the Train Tunnel, which is to the north-west of the first snowy open world section you find. There's a derelict train crashed outside, so it's not hard to spot.
Head inside by going up the path into the cave, then hop down at the other end to enter the dark tunnel. Walk to the back of the tunnel and when your objective changes to "search the train car", get ready for a fight because there's a bloomin' Carrier hiding behind it. Back off down the tunnel then whenever the Carrier stands on its hind legs and exposes its belly, shoot at it to deal the most damage. You can also shoot the hanging icicles in the cave to drop them onto its head.
When it's dead, head to the back of the tunnel where you found the beast chilling and kick down the door to the train cart. Inside, you'll find the Grindlift Maintenance Notes collectible and a component. Leave by climbing atop the train at the front of the tunnel and you're golden.
Gears 5 Hollow Storm: Old Derrick Site 1
You won't be able to visit the next two locations for the Gears 5 Hollow Storm mission until you've got through the gate and opened up the second area. When you do, head to the north-west corner of this section to find the first Old Derrick Site. Duck under the crane to enter and you'll be face to face with a bunch of Swarm enemies that haven't spotted you yet.
You can either go in guns blazing or start taking enemies out stealthily, but be aware that as soon as you alert them, a Warden will start making its way over to you. Therefore, I'd recommend stealthily killing as many enemies as possible so you can try to take down the Warden without having to deal with smaller enemies at the same time.
When you've taken all the enemies down, climb aboard the huge rusted vehicle and pry open the yellow cover to find the Grindlift Actuator Component. Leave via the way you entered and there's just one more location to go.
Gears 5 Hollow Storm: Old Derrick Site 2
Head east from the first Old Derrick Site and on the other side of a huge island in the middle of the ice is the entrance to the second Old Derrick Site. Enter and cross to the machine on the other side of the small ice lake, then interact with the yellow panel to start the Hollow Storm objective.
Enemies will start to attack from the area you entered from, and there's a lot of them. Stay in cover and slay as many foes as you can. Bear in mind that you can shoot the ice beneath their feet to trap them. When everything's dead, go back and pry open the panel for good this time to obtain the Grindlift Radiator Component. Now you've got all three parts, the Flash Freeze ability for Jack will be unlocked.