Thresher Maw

Description
Thresher Maws are subterranean carnivores that spend their entire lives eating or searching for something to eat.

They reproduce via spores that lie dormant for millennia, yet are robust enough to survive prolonged periods in deep space and atmospheric re-entry. As a result, thresher maw spores appear on many worlds, spread by previous generations of space travelers.

Thresher maws are solitary creatures; two or three at most can be found on a single planet. They live alone in nests spanning large areas underground. They can grow to be in excess of 30 metres tall above the ground, with a body nearly twice that size beneath the surface. The body of an adult thresher maw never entirely leaves the ground; only the head and tentacles erupt from the earth to attack.

Aggressive and highly territorial, thresher maws feed by absorbing huge amounts of solar radiation and survive best on planets, asteroids or moons with little or no atmosphere. They also consume minerals and ores from the ground itself and use an odd form of photosynthesis to combine solar radiation and minerals into energy they can use to fuel their significant mass.

Due to their size, and their burrowing style of movement, thresher maws are immobile above ground, but can move incredibly quickly below. They can take a lot of damage and can be very hard to kill. Their attacks consist of spitting powerful viscous acid that splashes on contact, burrowing up from beneath their prey, and smashing with their claws in close range while emitting infrasound. Due to the nature of their attacks, they completely ignore shields.

Source: Galactic Codex: Essentials Edition 2183

History
The Alliance first encountered threshers on the colony of Akuze in 2177.

After contact was lost with the pioneer team, marine units were deployed to investigate. The shore parties were set upon by hungry threshers, and nearly the entire assault force was killed.

Since then, thresher maws have been discovered all about the galaxy, surviving even severely hazardous environments.

Their physical and biological origins are unknown, although it is widely assumed they were "spread by a previous generation of space travelers", possibly before even the asari.

Among the survivors from Thresher Maw attacks on Akuze, serveral were experimented on by the black-ops Cerberus group. You may encounter a survivor of a Thresher Maw attack on Akuze in the mission UNC: Dead Scientists. If you choose 'Sole Survivor' during the psychological profile phase of character creation, Commander Shepard will have a history involving the thresher maws on Akuze as well.

Habitation
The thresher maw usually lives in large flat open spaces on uninhabited planets.

At least in the retail game, thresher maws do not inhabit rolling hills, mountains, valleys and perfectly flat land.

Specifically, thresher maw nests are uniquely identifiable by a distinct profile of landscape, which consists of a few hills and pits no higher or deeper than 2 meters on an otherwise perfectly flat circular area of land about 50-75 meters in diameter. Although barely discernible from radar, it becomes more apparent once it is seen from above. From below, it may appear as a jagged circular terrace. Thresher Maws will not rise from the ground upon the hills or pits in their nest. They will retreat below ground if you leave their circular area for longer than about 5 seconds.

Thresher Maws commonly have some sort of "lure" in their nest, but there are some "stray" nests, identifiable only by their landscape profiles.

This landscape profile may be a design decision to reduce development time to allow them to be "painted" onto maps in a single stroke, or more likely, it is to support their "rising from the ground" state of animation being correctly aligned with the surface of the terrain, and to ensure they have a radius in which they can attack without obstacle.

Tactics
Alliance forces recommend engaging Thresher Maws with vehicle-mounted heavy weapons.

When driving through thresher maw nests, be on your guard. They will abruptly rise from the ground after a short delay of having driven upon their nest. Once the noise of one rising up is heard, look for it and put some distance between yourself and it quickly.

You should only try to attack one from a distance, because their melee attacks are very devastating and rarely leave survivors. They have a sizable range beyond their appearance due to powerful infrasound. The ranged acid spit attack is fairly easy to dodge as it as slow, as well as less powerful in damage.

The quickest way to eliminate a thresher maw is circle-strafing and shooting the Mako's cannon at it. If you must stop to snipe, time your stop immediately after a spit has passed you by, or otherwise when the acid has a trajectory that won't hit you or your team. It a good idea to use the Mako's explosive canon as much as possible in combination with the coaxial machine gun, in order to produce the most damage.

If you kill a Thresher Maw while in the Mako, you are given significantly less experience. If you want to get the 'on foot' experience reward but still use the Mako's firepower, get the Thresher Maw down to as little health as possible using the Mako, then get out and finish it off on foot.

As the Thresher Maw is not invulnerable to any damage type, even your pistol can kill it. Use damage boosting skills as much as possible. Having Warp, or a party member with Warp, can serve to reduce the Thresher Maw's defenses considerably. Even if you can't see the Thresher Maw's life meter, or bullet impact effects on it, rest assured you are damaging it, if you are aiming at it.

Be certain to stay out of its melee range, as it is further than it physically appears to be. Don't foolishly attempt to determine it's melee range, just stay back. Keep enough distance between you such that you are able to determine the path the acid spit is taking. While circle strafing, your party members prefer to stop longer than you, and don't even try to dodge the incoming attacks, so try to shoot while continuing moving in a linear fashion. They may still run into acid spit aimed for you, and you may take splash effect damage off of them, so find a semi-circular pattern that avoids this.

Whether combating a Thresher Maw in vehicular or infantry combat, you should be aware of your odds of survival. When on foot, the acid's splash effect that may kill you, even if it doesn't hit you directly. High level soldier regeneration, and/or Medical Interface that provides Toxic resistance can mitigate this considerably. Surviving a direct hit from an acid splash requires a very high amount of health, as it completely ignores shielding. The Mako can be also substantially damaged or destroyed in as little as a single acid spit direct hit. If you are on foot, and your Mako is destroyed by acid spit, do not get back into it -- go to the map screen and return to the normandy instead.

Although when it comes to combat, there are always a lot of elements to take into consideration, the bottom line in this scenario is that as long as you keep circle strafing from a safe distance, you will rarely, if ever at all, be defeated. The Thresher Maw is not an overwhelmingly difficult opponent.

Oddities
On hardcore difficulty level, you get experience twice when killing a thresher maw - once when its severely injured and once again when it dies. It is unknown whether this is a bug or intended behaviour. In consequence, when playing on hardcore difficulty, do not mistake the first experience reward for an indication that the fight is over, or you might be in for a nasty surprise.

Unlike other enemies in the game, such as geth armatures, Thresher Maws do not have to be even close to looking in your direction to perform any of their attacks, so do not attempt to drive into or melee attack one.