Thresher Maws in Mass Effect are encountered on a number of uncharted worlds and can be fought in the Mako or on foot. 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.