Roughly halfway through the game, you and Wheatley overthrow GLaDOS and put him in charge in place of her. Almost immediately, Wheatley lets his newfound power get to his head, uploads GLaDOS into a potato battery, then throws her into the elevator with you and pounds it down into the ground, causing its support to give out and send both you and her falling miles down into a section of Aperture Science that time forgot. And were the company still in business at the time this game takes place, they'd probably rather you forgot about it as well.
Exploring the old, condemned Aperture facility, from the way it was in 1952 until the 1980s, allows the player to experience the journey Aperture, and its founder and CEO, Cave Johnson (played by the spectacular J.K. Simmons), took in getting the company started up as a shower curtain manufacturer, experiencing success in science innovation, and their fall from grace when the true nature of their experiments came to light. Experiments like turning someone's blood into gasoline, and having a superconductor pointed at someone (resulting in getting a tumor or best case scenario, superpowers). Oh and apparently making lemonade was planned as well at some point. But the plan was scrapped.
It's also revealed in Johnson's rant how exactly GLaDOS was brought into being. It's because of the rich lore the old facility contains about Aperture as a whole that it's one of my favorite moments in the game. It is, for all intents and purposes, a history lesson, and one that I actually enjoy. Kind of sad, really.