Burrow is like The 500 Hats of Bartholomew Cubbins—it doesn’t have infinite uses, each more elaborate than the last, but it does have more than you expect. At the same time, burrow is like the hats in that is it not often practical. It’s fairly rare to research burrow in pro games, though there are situations where it is common.
The Liquipedia article on burrow is fairly thorough, though I know more than it does. I’d like to organize the information differently, by the tactical or micro purpose. Of course, it’s common for one use of burrow to have more than one purpose. As far as I can tell, my list of purposes is exhaustive. Did I miss anything?
First, an observation: Many bots don’t get detection unless and until they know that they need it. Also many bots don’t recognize or react to burrowing when they see it (though Steamhammer does). In bot play, I think burrow is highly abusable, in other words it has a strong chance to exploit the opponent’s weaknesses.
Also, coordination always helps. If the enemy is camping your burrowed units to keep them down, maybe you can pressure somewhere else to release them. If you’ve laid a burrow trap, maybe you can lure the enemy into it, or force detection somewhere else to distract. And so on.
To hide for safety. When a reaver comes in to drop, there is probably no observer around. Drones can keep mining until the reaver appears on the ground, then burrow before they are targeted. (It’s not good against storm drops, though.) If outnumbered units can’t get away (they are slow or are cornered), they may be able to burrow and hide; at worst, the enemy will have to make extra effort to detect them, and at best they may live. If you think the enemy doesn’t have detection, you can burrow almost any time that you don’t want to fight. It may complicate the enemy’s tactical reasoning. Of course the enemy could choose to stand around on top of your burrowed units until they can be dealt with, so it may also complicate your tactical reasoning.
To hide in ambush. This is something I tried as a beginner: Burrow zerglings on the map where marines will pass by, and when they unsuspectingly walk over, unburrow for a surprise attack. It’s hard to do successfully, and the few times I’ve seen it tried in pro games it usually fails, though in the best case it can be devastating. It’s easier to burrow outside the enemy base for a backstab after the enemy army leaves. Or burrow in position for a sandwich; for example, burrow outside your base and unburrow when the enemy approaches your sunkens.
Against corsair-reaver play, it is common to burrow hydralisks in places where shuttles may appear. Ideally, you can snipe shuttles before any reavers land.
You can burrow near a base location that the enemy should take, and wait for the base to start construction, or to finish construction, or even for workers to arrive, before springing the ambush.
An infested terran does tremendous damage, and an infested terran can burrow. (That’s some extreme infestation.) Infested terran burrow traps are tempting. If terran is making mass tanks, maybe there’s not much gas left over for vessels and you can burrow in position to wipe out clustered tanks. 3 infested terrans kill a command center. You can try to burrow them where a base will be. If the queen is still around and you want another infested command center, you only need 2 infested terrans. But in most situations, probably drop is more practical than burrow for deploying infested terrans.
Hiding has many tactical uses. If you are short of overlords near the enemy base (perhaps on purpose so they’re not spotted), you can elevator a force piece by piece into an unseen corner and keep them burrowed there until the attack force is complete. It’s not a skill worth implementing in a bot, but I want to show that creative uses are out there.
To gain vision. Also common in corsair-reaver is to burrow zerglings around the map to reveal where the enemy is flying. It’s especially useful in corsair-reaver, because observers do not come along so the enemy rarely finds the burrowed lings. It’s potentially useful whenever the enemy moves around without detection. You can burrow zerglings at chokes, or in between bases to catch when workers transfer.
You can burrow in the enemy base to watch. A scout that survives until burrow is researched but then is in danger of being caught—burrow. A zergling force sneaks in to pick at probes, but 1 burrows by the gateways to see what comes out, or near the ramp to see what passes by. If you see the robo, maybe you can find a spot away from the path that observers will take.
To block building construction. Some terran bots place spider mines at every base location. A zergling works just as well for gaining vision and for blocking construction, and it has the option of unburrowing to chase off the enemy worker, or to look around the area, or to flee if in danger. Because it can unburrow, it has a chance to spot the start of the enemy base, rather than just the presence of the enemy.
It’s theoretically possible to use burrow to spoil a wall, or to prevent a terran addon from being made. I find it hard to imagine when that would be useful. But consider: Many bots are unable to adapt when they fail to place a building, and simply try to place it in the same location again. A burrowed unit anywhere that one of these bots tries to build may mess it up severely.
To take advantage of stacking. Burrowed units stack, somewhat like air units. In other words, units under the ground never collide with each other. You can burrow arbitrarily many units in the same patch of ground, one group after another. You can make a large force look like a small force, even to a detector, by burrowing units on top of each other. If you stack lurkers on top of each other, the lurkers will probably fire at the same time at the same target and their damage will also stack. Strong players do this all the time, to make it unclear how many lurkers there are, to make it hard to irradiate all the lurkers, and to get the effect of the stacked damage.
Units above the ground of course do collide when their collision boxes overlap. When you unburrow units which overlap with other units you are unburrowing, or with units already on the surface, then the units don’t all fit in the space they occupy. They get stuck on each other and start to glitch and spread out. Sometimes that’s bad; it causes awkwardness all the time for Steamhammer when unburrowing lurkers. There are also ways to exploit the collisions.
The Hoejja build is a way to exploit the collisions. It uses burrow and unburrow to glitch early zerglings through the blocking mineral patch on some maps—not all the lings get through, but it can be enough. It relies on the opponent to play a build with no early mobile defense, namely protoss forge expand. On the one hand, the only SSCAIT map suitable for this trick is Destination. On the other hand, a bot should be able to do the burrow-unburrow trick much faster and more precisely than a human, and a machine opponent is unlikely to understand how to react.
You could try the same burrow-unburrow trick to pass units through a wall. Does it work against Iron?
Another way to exploit the collisions is to spring a burrow trap which causes the enemy units to glitch. If other forces attack at the same time, it won’t matter at all that some of your unburrowing units are glitching too.
To prevent spell effects. Burrowed units avoid some spells with area effects.
These spells leave burrowed units unaffected: Maelstrom, stasis, recall, ensnare, plague. Of course recall only works on your own units (which you must have gained by mind control if they can burrow), so inability to recall a burrowed unit is a limitation. (I imagine that the limitation is because burrowed units stack; the designers don’t want you to be able to recall an arbitrarily large zerg force.) For the other spells, you may be able to avoid the effect by burrowing early enough.
These spells affect burrowed units as usual: EMP, defensive matrix, yamato cannon, hallucination, psionic storm, disruption web, broodling, parasite. The visual effect of defensive matrix is not visible on a burrowed unit. Disruption web is mostly useful for disrupting burrowed units which are lurkers, because others can’t attack while burrowed. If you dweb burrowed zerglings, they can unburrow and leave the area of effect to attack.
You can broodling a burrowed unit. The 2 broodlings appear above ground and cannot burrow. (Liquipedia says that all ground units other than ultralisks can burrow. They forgot about broodlings.) You can hallucinate a burrowed unit. The hallucinations appear above ground and cannot burrow.
Not on either list above is irradiate. You can irradiate a burrowed unit as usual. But the splash damage of irradiate is absorbed by the ground: It does not splash from or to any burrowed unit. If one hydralisk in a group is irradiated, you can burrow it to save the others. Or you can burrow drones when terran tries the eraser trick, flying an irradiated science vessel overhead. Both ways prevent irradiate from causing splash damage.
To prevent splash damage. Burrowed units avoid some splash damage. Liquipedia explains it as a splash damage effect of dark swarm, but dark swarm is another topic. As far as I know, the only important splash avoided is tank splash. I didn’t test it to find out, but it’s possible that they also avoid the outer ranges of nuke damage.
If you set up an array of zerglings and have a tank fire once at the central ling, then tank splash works like this: The central ling and some around it are killed, while others a little farther away are damaged. If you burrow the zerglings and then have the tank fire once, then the central ling and some around it are killed, while those a little farther away are unharmed. The splash damage was eliminated outside an area of central effect.
Under dark swarm, the tank shot is offset from its aim point. It still kills zerglings in its area of central effect, so the terran may be able to analyze the pattern of units under dark swarm and aim at one that puts the actual impact point where desired. Or zerg can arrange the units so that important units cannot be hit by aiming at another unit. When I tested it, I found some unexpected tank splash outside the impact point, so it may be that dark swarm has more complicated effects. At least, there’s probably something I don’t understand.
To prevent slow effects. I haven’t tested this, but it seems likely to work. Some attacks, like the reaver scarab and the ensnare spell, are slow and you can see them coming because they have a “bullet” that has to travel to the target. Burrowing takes time too, but it is fast. I think you can probably avoid slow effects in some cases by burrowing immediately.
I thought of another tricky idea to stop reaver shots using burrow. It’s not practical, but it’s cute: Burrow a tight line of units in front of or around a target (likely across a choke). When the reaver scarab comes, you should be able to unburrow in time to block the shot. I expect the scarab would be unable to find a path to the target and would dud out. Then you can burrow again before the next shot. If you want it to look impressive, predict the scarab’s path and unburrow just the units needed to block it at each moment, like playing Pong with a scarab. (Seriously. Somebody should try to code scarab Pong, just for fun.)
The hold lurker trick. This isn’t about burrow specifically, but it is common in human play and it is related. It’s worth a mention. See the Liquipedia article for details.
