The Hidden Workshop is a location in the world of Ninjago. It is Dr. Julien's underground laboratory and former home, used for creating robots and other inventions. This is where Dr. Julien built Zane, the Juggernaut, and the Falcon, and where he ultimately passed away.
The Hidden Workshop is built under the ground of Birchwood Forest, with its only entrance hidden in a birch tree. The only sign of the entrance is a symbol on the trunk, used by Dr. Julien to mark all of his creations.
The Falcon attempted to lead the Ninja to the Hidden Workshop, but only Zane had the stamina to keep up with it as they arrived in Birchwood Forest. When the Falcon malfunctioned, Zane came upon the workshop, defeating the Juggernaut in battle when it attacked him. Inside the workshop, Zane found a set of blueprints which allowed him to repair the Falcon, but also discovered his own blueprints.
When the other three Ninja found the Hidden Workshop, Zane revealed his true nature to them. Leaving their comrade to contemplate the revelation, they exited the workshop only to find it besieged by Treehorns. Meanwhile, Zane activated his "Memory" switch, discovering memories of his creator, Dr. Julien, who had built and raised the Nindroid in the workshop. Zane also learned that Dr. Julien had died in the workshop, but not before reaffirming his love of his artificial son and turning off his Memory switch to spare him the pain of losing a loved one. Shedding a single tear for his father/creator, Zane set out from the Hidden Workshop to aid his friends against the Treehorns.
When Zane miraculously reunited with Dr. Julien on a small island guarded by the Leviathan, the scientist revealed that Samukai had broken into the Hidden Workshop shortly after Zane first left. Using a mysterious potion to revive Dr. Julien, Samukai forced him to build weapons and vehicles for the Skulkin Army.