The Alpha Warhead is a Mechanic in SCP: Secret Laboratory that causes the underground section of Site-02 to blow up, forcing everyone to evacuate to the Surface.
The Alpha Warhead acts as a last resort emergency method in an event of a serious containment breach, with the intent of blowing up any threat that is inside Site-02.

In Game

The Alpha Warhead is a mechanic used to destroy the underground sections of Site-02, alongside all living beings present inside. It is intended to prevent prolonging of rounds, forcing all participants to Surface Zone post-detonation to bring a swift end to the round.

Enabling

To turn on the Alpha Warhead, one most locate the silo in Heavy Containment Zone and flip its lever into the "Ready" position. This can be done by anyone except SCP-079.

Afterwards, one must go up to the Surface Zone and enter the Warhead Control Room. Inside, there is a button locked underneath a glass panel, which requires a Keycard with Nuke Access

Nuke Access

to open. Afterwards, the button can be pressed by anyone if the Alpha Warhead is enabled.

The screen on the terminal will state the current status of the Alpha Warhead. "Ready" means you can turn the Alpha Warhead on. "Disabled" means you cannot turn the Alpha Warhead on. "Please Wait" means that you must wait before turning the Alpha Warhead back on.

Initiating

If the warhead is enabled, and the button is pressed, a countdown will begin on the screen in each of the Warhead Rooms. A loud alarm will also play twice as soon as the detonation sequence has begun. C.A.S.S.I.E. will then announce the activation and how much time until the Alpha Warhead blows up.
The time until detonation can range from 80 seconds to 120 seconds depending on the server, though the default time is 90 seconds. During the detonation sequence, a siren will begin to play, and red lights will turn on throughout the entire facility, including the Surface.
All doors within the facility will be locked in an open state during the nuke, with the exception of SCP-106's Containment Chamber Door, and SCP-079's Containment Chamber Door, which will stay closed.
Servers can use also configs to make it possible for Gate A and B to be closed during the detonation sequence; by default, Gate A and B will be forced open during the detonation sequence.

During the last 45 seconds of the timer until detonation, a snippet of "Ajoura - The SCP Foundation Main Theme" will begin to play.

The Overcharge event will not force doors shut during the detonation sequence.

Cancellation

To cancel the detonation sequence, a red button located in the Alpha Warhead Silo on the control panel must be pressed. When this happens, the sirens will stop, the lights go back to normal, and C.A.S.S.I.E. will announce the warhead has been cancelled. The lever can also be flipped back to the off state to stop people from turning the Alpha Warhead back on, forcing people to go back to the Nuke Silo to flip it back on.
If the detonation sequence is started again after being cancelled, C.A.S.S.I.E. will announce that the sequence has been resumed, and will say how much time is remaining. The time remaining is based off how much time was left when the Alpha Warhead was last canceled.

It is impossible to turn off the Alpha Warhead at the last 10 seconds of the countdown; the screens in both the Silo and Surface nuke room flash "Detonation Inevitable" above and below the countdown.

Detonation

When the Alpha Warhead blows up, anything left inside the facility will be instantly killed. This includes SCP-079, regardless of what camera it is looking through (making it one of the three ways to kill SCP-079). The door to the Nuke Room on Surface Zone will also be forced open and cannot be closed.

Upon detonation, the next respawn wave will be forced to a four minute cooldown, regardless of their timer before the detonation. This means that if a team is spawning, for example the MTF Helicopter is about to land but the Alpha Warhead detonated, it will simply not spawn the MTF and go back.