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The Rollermine is an electrically charged spherical device, created by the Combine. Slightly larger than a football, it reacts to enemy movement within a sizable radius, and uses electrical shocks to subdue its targets.
Rollermines are used as offensive and defensive traps by the Combine. As the name implies, they roll towards enemies and electrocute them via close-range electric shocks. Extensively used along the coastal roads, they are deployed from the air by Combine Dropships, in groups of two to four. Upon landing, the mines will remain inert or, if they land on softer terrain like soil or gravel, they will burrow themselves into the ground, remaining almost invisible. When an enemy target is detected, the roller mine will pop out of hiding and roll towards it in a relatively straight line.
The mines can magnetically cling to a target vehicle and interfere with the steering, forcing the driver to either stop or drive off the road. Besides the damage inflicted by any resulting car wreck, attached Rollermines do not directly injure the occupants of a vehicle. For this reason, Rollermines are usually deployed in tandem with Combine soldiers, who ambush the stranded individuals. Attached mines cannot be removed except through excessive force, chiefly the Gravity Gun; touching them will merely cause electrocution. They are also deployed in buildings and structures, taking any intruder by surprise. Rollermines can be destroyed with explosive devices, or alternatively may be picked up with the Gravity Gun and launched out of the way (or at enemies). Complete submersion in water also causes them to short-circuit and explode. Additionally, one can elude Rollermines by outrunning them, as they cease chasing once a target is too far away. If Rollermines are caught in an explosion, they bounce up and detonate.
Alyx was at one point able to capture and disarm a Rollermine, which she then gave to her "pet" Dog to play with, calling it a ball. Alyx had Dog use the "ball" to play catch with Gordon Freeman so Freeman could get practice using the Gravity Gun.
In Episode One, Alyx reveals that she can use her EMP Tool to temporarily reprogram a Rollermine's targeting system, making it "friendly". Once reprogrammed, Rollermines will emit an orange light instead of their signature blue and will follow the player slowly, attacking any Combine units it sees (including other Rollermines). Reprogramming Rollermines makes them unstable, however, and they will invariably emit red light and explode a few minutes later.
 Related Achievements
|Two Points (2G)|
|Use Dog's ball to make a basket in the scrapyard.|
 Behind the scenes
The Rollermine model met in the playable Half-Life 2 leak looked significantly different, bearing some resemblance to a sea mine and was more aggressive; attacking the target constantly, whereas in the retail version when it attacks it has to charge before attacking again.
- Rollermines can attach to the Dune Buggy and shock it - strangely, so will Dog's ball as if it was still active.
- Rollermines can also be destroyed by placing them in a fire.
- Reprogrammed Rollermines seem to do varying damage to different targets. They can take out Overwatch Soldiers (50 HP) in two hits, and take out an Overwatch Sniper (125 HP) in one hit.
- In Half-Life 2: Episode One during the scene where the sniper is first seen, even if the player only lets Alyx reprogram one Rollermine, all of the Rollermines in the area will explode once the sniper is killed as if the other Rollermine is reprogrammed.
 Dog's Ball
- It is possible to take Dog's ball or active Rollermines through Ravenholm to Nova Prospekt, but invisible walls and glitches that have to do with friendly NPCs attacking it make it a very tough feat. There are also environmental challenges such as the other Rollermines, and having to go underwater, which will destroy Dog's ball if it is submerged too deeply. The upside of keeping the ball is that it is easy to kill Antlion Guards and hostile antlions, headcrab zombies, and individual headcrabs. It also proves as a great distraction for Overwatch soldiers by holding it with the gravity gun, and can also be used as a shield or a projectile. It is also possible with Manhacks.
- If Dog's ball is taken through Ravenholm and through the mines there will be an invisible wall blocking the way to Shorepoint which will prohibit the Rollermine's entry to Shorepoint. However, if launched with the Gravity Gun high enough, it will go over the wall and the player will be able to enter the base and the later chapters. This tactic can be repeated for the three or more invisible walls later on. At the end of Sandtraps, the Rollermine can be placed outside the entrance. This will cause the ceiling to crumble, blocking the player from coming in but will allow the Rollermine (deactivated or active) to enter, despite it the debris.
- Dog's ball will react to the G-man's presence outside Shorepoint but will do relatively nothing but follow him.
- If one can get Dog's ball to N.L.O and place it in front of the Vortigaunt under one of the tables, he will randomly say all of their quotes for when he is done charging Gordon's suit.
Overwatch Soldiers attacked by reprogrammed rollermines on the Citadel train platform.
 List of appearances
- Half-Life 2 (First appearance)
- Half-Life 2: Episode One
- Source Particle Benchmark (Game files only) (Non-canonical appearance)