Secret Headquarters: The Sentinel's Anthill

from Avengers #104 (October 1972)

Clipped from uncannyxmen.net: 

Around the same time, the other Sentinels were still orbiting around the sun. Number Two, the leader of this pack of Sentinels even flew so close to the sun that his hide began to melt. Additionally, his systems were altered on a molecular level, with Number Two somehow gaining the power to open teleportational gates. Once the Sentinels realized that they lacked the means to destroy the sun, they came up with another plan to end the mutant threat. As they couldn't end the cause of mutant births in general, they saw no other way but to control each single birth that occurred. In order to do so, they retreated to a secondary base they had built located somewhere in the Australian Outback and resembling a giant ant-hill. There, the Sentinels built a device that would increase solar flare activity up to a point where it would not kill humanity but render them sterile. Afterwards, the giant robots planned to genetically create a new human race in their labs – not caring whether the task would take a few years or even centuries.

...the mutant energy detector in the command room was switched off. When the detector was turned on, it bathed every mutant in close proximity in a glowing light. The Sentinel Number Two also went aglow – the flight around the sun having altered his make-up so much that he now qualified as a mutant.  Immediately the other three remaining Sentinels turned on their leader, obliterating Number Two with their in-built lasers and ray guns. Apparently still connected to Number Two in some way, the Sentinels abruptly deactivated themselves right afterwards instead of carrying on the fight and their plan to render humanity sterile.  The Avengers caused a cave-in to seal off the entrance to the anthill, before returning home.