Diggernaut

The Diggernaut is a boss that appears in Metroid: Samus Returns. It was once a mining robot used by the Chozo race to expand their colony on SR-388, but since the Chozos abandoned the planet, the Diggernaut continued to dig throughout the caverns, eventually going rogue from a lack of maintenance.

First Encounter
In Area 3, Samus will encounter a lone red light while exploring the subterranean cave system for Metroids. Believing it to be a Grapple Point, she uses her Grapple Beam to pull on it. This will give the Diggernaut a surge of electricity and reawaken, clearing the debris blocking Samus' path. The Diggernaut briefly stares at Samus before walking away.

Chase Sequence
Latter in Area 4, Samus discovers a destroyed Chozo statue with its Item Sphere removed. After investigating for a few rooms, she encounters the Diggernaut, seeing her as a threat, uses one of its drills to chase Samus down a series of tunnels.

Touching the Diggernaut's drill will kill Samus instantly, so she will have to do her best in order not to be rammed into them. Several obstacles and enemies are located in the tunnels and will attempt to distract Samus from avoiding the drill.

Eventually, Samus will come across a single hole in a wall at a dead end, and makes her way in there through the Morph Ball, narrowly avoiding the drill from crushing her. Presuming her to be dead, the Diggernaut extracts its arm and spits out the Item Sphere. Samus can then collect the Item Sphere and obtain the Space Jump.

Battle
In Area 6, before Samus can grab the Item Sphere obtaining the Power Bombs, the Diggernaut appears and vacuums up both the Item Sphere and the Chozo Statue. Later in the Area, Samus will come across a dark room with a large metallic door at the end of it. The door opens revealing the Diggernaut, ready to face her itself.

The Diggernaut uses its drills to drag along the floor and foreground, which Samus can avoid with a Space Jump. Eventually, it will tilt its head forward and shoot out two lasers from each side of its eye. Shooting the faceplate enough times will stun the Diggernaut and bring its drills forward. When approaching a drill, Samus has to use the Spider Ball on the side of the drill with Spider Ball tracks or risk damage. Once on the inside, Samus has to wait until the drill turns to the side revealing the Bomb Slot, in which Samus uses a Bomb to destroy.

The Diggernaut escapes from the stun and uses its drills on the ceiling, in which multiple purple blobs rain below, though Samus can blast them and obtain Energy and Missiles. The Diggernaut will aim a laser at Samus as she fires at the blobs, though only firing should Samus be still. Afterwards, the Diggernaut will use its vacuum to suck up Samus, dealing three tanks of Energy of damage should she fall in, though it is possible to simply jump or use the Spider Ball to avoid being pulled in. The premise is to lay Bombs for the Diggernaut to vacuum up. The Diggernaut will then fire more lasers from its eye, three compared to two from before. After enough damage is dealt to the faceplate, Samus bombs the other drill, disabling them both and moves the battle to the second phase.

The Diggernaut uses its damage slam as blunt weapons, slamming them onto the floor thrice, creating a small shock wave. It will then fire eight lasers from its eyes in two sets of four, with gaps in-between for Samus to jump through. It will begin to use its vacuum attack again, in which Samus must use Bombs in order to continue on the fight. After sustaining enough damage, the Diggernaut's faceplate will come into the foreground, lined with Spider Ball Tracks and featuring three weak points. Samus must attach herself to the faceplate with the Spider Ball and lay Bombs on each weak point, hurrying before the faceplate generates electricity to detach her. However, the weak points are rotating clockwise and must match with a line on an inner layer, which is rotating counter-clockwise in order to deal damage. The Diggernaut will resume its previous attacks after some time. Once all three circuits are severed, the Diggernaut collapses and drops the Item Sphere.

As Samus walks towards the Item Sphere, the Diggernaut is still functional and attempts to crush Samus while she is distracted. However, Samus loses her patience, and uses her Plasma Beam to blast off the Diggernaut's head, disabling it for good. Using a Power Bomb will obliterate the rest of the machine and unblock the path.

Official Media
"The Diggernaut is an ancient Chozo machine created to dig out the caverns of SR388, giving the Chozo more space to build their settlement. The Diggernaut has since malfunctioned, or was set with some sort of security protocol. Whatever the truth is, the Diggernaut serves as a thorn in your side during most of your mission."

- Metroid: Samus Returns Official Guide

Trivia

 * The Diggernaut is one of the two bosses featured in Samus Returns that did not appear in the original Metroid II: Return of Samus, the other being Proteus Ridley.
 * The Diggernaut's name is a portmanteau of two words: "digger", referring to its purpose to mine, and "juggernaut", referring to its massive size and power.
 * In the Chozo memories image and before the boss battle, several small robots similar in appearance to the Diggernaut accompany the machine, though oddly none of them are fought alongside the Diggernaut, or in the entire game.
 * The Diggernaut is the only boss in the game that cannot be Melee Countered.
 * The Spider Ball tracks on the Diggernaut may be a homage to the Metroid Prime series.
 * The battle against the Diggernaut is also quite similar to the boss battles with Quadraxis and the second phase of Emperor Ing from Metroid Prime 2: Echoes.