In 2001, Apple introduced Mac OS X, based on Darwin and NEXTSTEP; its new features included the Dock and the Aqua user interface. During the transition, Apple included a virtual machine subsystem known as Classic, allowing users to run Mac OS 9 applications under Mac OS X 10.4 and earlier on PowerPC machines. Apple introduced Mac OS X 10.8 in February, and it was made available in the summer of 2012. Mountain Lion includes many new features, such as Mission Control, the Mac App Store (available to Mac OS X v10.6.6 "Snow Leopard." users by software update), Launchpad, an application viewer and launcher akin to the iOS Home Screen, and Resume, a feature similar to the hibernate function found in Microsoft Windows. The most recent version is OS X El Capitan . In addition to Mavericks, all new Macs are bundled with assorted Apple-produced applications, including iLife, the Safari web browser and the iTunes media player. Apple introduced Mavericks at WWDC 2013 in June, and released it on October 15 of that year. It is free of charge to everyone running Snow Leopard or later and is compatible with most Macs from 2007 and later. Mavericks brought a lot of the iOS apps, functions, and feel to the Mac as well as better multi display support, iBooks, Maps, app nap, and other upgrades to improve performance and battery life.

What was Apple's Mac OS X based on?