Points of Interest
The Infinite Avengers Mansion appears to stretch on forever and at each mile marker there is a version of Jocasta to help anyone who is lost, but there are a limitless number of doors in the mansion which could lead to anywhere – including other dimensions or planets. If there is somewhere the mansion doesn't connect to, Pym can create a door to go there.
Mighty Avengers #27 (2009)
After Loki adopted the identity of the Scarlet Witch, he brought together a team of Avengers led by Henry Pym, now calling himself the Wasp after his deceased wife, Janet Van Dyne. This got Pym thinking about the concept of magic as related to science – quantum physics specifically – and that's how he came up with the idea of the Infinite Avengers Mansion.
The mansion became headquarters to his team, and it seemed to go on forever. The most concerned was the Avengers' butler, Jarvis, until he found out it was self-cleaning. In addition, Pym installed an infinite number of doorways that could lead anywhere – from familiar places on Earth to the home of the Inhumans in deep space. If the mansion didn't have a door for every occasion, Pym could always create more. If anyone became lost within its confines, there was a replica of Jocasta at every mile marker to assist with directions. Once outside the mansion, the only way to get back in was with a subatomic key implanted into the palm of whichever individual Pym allowed access to his creation.
While Loki remained close to the team to manipulate them for his own evil purposes, Stature was the only one to know the truth. She couldn't tell the other Avengers as Loki placed an enchantment on her, but she knew she could always rely on the Young Avengers for help. If that wasn't enough, Ronin tracked them back to the Infinite Mansion and a fight ensued, ending with Loki's deception uncovered and his defeat.
After returning from another assignment, Pym brought along an unseen stowaway – a tiny robotic vessel carrying the essence of Ultron. The maniacal automation implanted itself inside one of the Jocasta replicas and continued to infect the rest until his body could be restored. Once back in his true form, Ultron wasted no time in corrupting the mansion's systems and reshaping it in his image. Resistance was limited as the Avengers had just disbanded and only Pym, the original Jocasta and two members of G.R.A.M.P.A., Ace and One-Eyed Jacquie, were present. Using the ten billion female impostors as his army, Ultron attacked. Pym quickly disabled the heart of the mansion and stored it within Jocasta's chest, which cut it off from the rest of reality, preventing Ultron from invading any part of the universe he saw fit. Pym sealed himself and the others in his lab, giving Ultron pause, and that is when the true location of the Infinite Avengers Mansion was revealed – Underspace.