This morning, if you opened your browser and went to NYTimes.com, an amazing thing happened in the milliseconds between your click and when the news about North Korea and James Murdoch appeared on your screen. Data from this single visit was sent to 10 different companies, including Microsoft and Google subsidiaries, a gaggle of traffic-logging sites, and other, smaller ad firms. Nearly instantaneously, these companies can log your visit, place ads tailored for your eyes specifically, and add to the ever-growing online file about you. There’s nothing necessarily sinister about this subterranean data exchange: this is, after all, the advertising ecosystem that supports free online content. All the data lets advertisers tune their ads, and the rest of the information logging lets them measure how well things are actually working. And I do not mean to pick on The New York Times. While visiting the Huffington Post or The Atlantic or Business Insider, the same process happens to a greater or lesser degree. Every move you make on the Internet is worth some tiny amount to someone, and a panoply of companies want to make sure that no step along your Internet journey goes unmonetized.

But though doctors have been putting people under for more than 150 years, what happens in the brain during general anesthesia is a mystery. Scientists don’t know much about the extent to which these drugs tap into the same brain circuitry we use when we sleep, or how being anesthetized differs from other ways of losing consciousness, such as slipping into a coma following an injury. Are parts of the brain truly shutting off, or do they simply stop communicating with each other? How is being anesthetized different from a state of hypnosis or deep meditation? And what happens in the brain in the transition between consciousness and unconsciousness? “We know we can get you in and out of this safely,” Brown says, “but we still can’t quite tell you how it works.”