Mega Upgrades are a thing that can be bought from the Square Shop of diepiness. They are upgrades which take you to Level M, a secret level which can only be accessed with this. They can be activated when in danger only and tend to be the most overpowered classes in the game.
Mega Upgrades are only able to be activated once per server. They set your Skill Points to a specific set for each Mega. Instead of HP, they have a timer which drains away gradually (30 seconds to drain by default) and is affected by HP factors too. When the upgrade is available, a pink glow will appear around the Tank. When Upgrading or De-uphrading, any Tanks in the glow will be insta-killed (except Smashers, which will only be blown back a lot).
List of Mega Upgrades
Mega Tank: A Tank with a double-firing Tank barrel inside another one on the top, bottom and both sides.
Mega Flank: A Flank Guard with Destroyer cannons on all four sides and a four-gunned Auto Turret on top.
More detailed coverage soon.