Thresher Maw

Thresher Maws are subterranean carnivores that spend their entire lives eating or searching for something to eat. They are enormous, violent creatures that burst up from the ground without warning when disturbed.

They reproduce via spores that can lie dormant for millennia and 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.

Description
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 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 ground to attack.

Aggressive and highly territorial, thresher maws feed by absorbing significant quantities 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 for use in an unusual form of photosynthesis. This metabolic process, utilizing solar radiation and minerals as raw materials, creates the fuel that powers their various biological processes and allows them to move their significant mass with surprising speed.

Due to their size, and their burrowing method of movement, thresher maws are mostly immobile above ground, but can move at incredible velocities through subterranean environments. They can absorb a tremendous amount of damage and are very hard to kill. Their attacks consist of spitting a powerful, viscous acid that splashes on contact, burrowing up from beneath their prey, and smashing their targets with their claws in close range while emitting infra sound. Due to the overwhelming, direct physical nature of their attacks shields are completely ineffective at mitigating any damage that is taken.

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 throughout 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.

If Commander Shepard has the Sole Survivor background, Shepard alone survived the thresher attack on Akuze. However, Shepard may later encounter a second survivor in the mission UNC: Dead Scientists, Corporal Toombs, who claims that a black-ops group called Cerberus caused the massacre at Akuze deliberately. He says Cerberus scientists wanted to study the thresher maws, so they let them kill not only the colonists, but the marine rescue team as well, to watch and study. Cerberus was delighted that Toombs survived and held him captive for years, using him for experiments. It is likely Toombs' story is true, as Cerberus used a similar technique six years later, luring Admiral Kahoku's marine team to their deaths in a thresher nest on Edolus.

Habitation
Thresher maws usually live in large flat open spaces on uninhabited planets. Thresher maws do not inhabit rolling hills, mountains, or valleys.

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.

Threshers commonly have some sort of "lure" in their nest, such as a crashed probe to draw unwary scavengers, 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.)

Mass Effect
Thresher Maws are encountered on a number of uncharted worlds. There is usually only one per planet, but some have two. They are usually encountered in the Mako, but can be fought on foot as well. The Maws that are encountered can be bypassed if necessary. Planets known to have Thresher Maws are Antibaar, Chohe, Nepheron, Nonuel, Presrop, Xawin, Trebin, Edolus and Luna.

Mass Effect 2
Only one Thresher Maw is encountered in Mass Effect 2, on Tuchanka during the mission Grunt: Rite of Passage; it must be fought on foot. While it is possible to defeat the Thresher Maw, earning respect among the krogan, this section of the mission can be completed just by surviving.

Mass Effect 3
In Mass Effect 3, the only thresher maw encountered is Kalros, "the mother from which all other thresher maws spawn." Kalros is the largest thresher maw yet seen.

She is summoned to attack a Reaper Destroyer that is guarding the Shroud on Tuchanka. After Shepard activates the maw hammers, Kalros attacks and defeats the Destroyer by coiling around it and pulling it into the ground, clearing the path to the Shroud.

Trivia

 * Thresher maws appear to be inspired by the giant sandworms of Frank Herbert's Dune. This is somewhat substantiated by the use of maw hammers in Mass Effect 3 to summon Kalros. Similar devices called "thumpers" were used to attract sandworms in the Dune novels.
 * The Thresher Maw may also be inspired by the Sarlacc, a creature present in the Star Wars universe, as the two bear many similarities, particularly in terms of interplanetary migration.
 * It is also possible that the Thresher Maw, especially the accounts of attacks on marine units, may be inspired by the opening scene of Gunman Chronicles, in which squads of soldiers and a fighter-class starship are set upon by a pair of plant-like creatures similar to, if much larger than, Thresher Maws.
 * If the Thresher is slain during Grunt's Loyalty mission, Gatatog Uvenk will inform Shepard that no krogan has slain a Thresher Maw on foot in generations; the last one to accomplish this feat was Urdnot Wrex. EDI will inform Shepard that defeating the Thresher Maw has generated several breeding requests for Grunt and, humorously, one for Shepard. Two krogan can be heard commenting that they'll be "Eating Thresher steaks for months".
 * According to Urdnot Bakara, a female krogan shaman, Kalros is the "Mother of all Thresher Maws", and that all Thresher Maws were spawned from it. Within the ancient ruins Shepard has to navigate through there is a mural of Kalros.
 * According to Javik, Thresher Maws were once small enough to be ridden.