Some U-M professors started the fall semester with classes enrolling tens of thousands of students. But no one had to cram into a campus lecture hall. Instead, they’re enrolled in “Massive Open Online Courses,” or MOOCs–the hottest educational sensation of 2012.

More than 1.6 million people from all over the world have signed up for nearly 200 free, non-credit MOOCs offered by thirty-three universities through the online website Coursera.org, founded by Stanford University computer science profs Daphne Koller and Andrew Ng.

The U-M debuted its first three MOOCs this past summer: “Internet History, Technology, and Security,” taught by Charles Severance, “Fantasy and Science Fiction: The Human Mind, Our Modern World” by Eric Rabkin, and “Introduction to Finance” by Gautam Kaul. Kaul says he found it “quite challenging and rewarding: challenging because it has forced me to teach to no one, yet everyone. Rewarding because we can reach so many people”–more than 125,000 students registered for his class.

Kaul says it’s “too early to tell” whether online education is as good as traditional brick-and-mortar classes. “It certainly can and should complement traditional education,” he says. “It also will hopefully provide access to people who do not have the time and resources to participate in high-quality traditional education.” Also still to be determined is a sustainable financial model; one possibility Koller and Ng have mentioned is a tie-in with online job placement services.

In October, 15,000 people had signed up for Alex Halderman’s “Securing Digital Democracy,” 25,000 for Silvio Savarese and Fei-Fei Li’s “Computer Vision: From 3D Reconstruction to Visual Recognition,” 57,000 for Lada Adamic’s “Social Network Analysis,” and nearly 92,000 for Scott Page’s “Model Thinking.” But unlike the brick-and-mortar students, most soon drop out. “It’s hard to put a finger on ‘enrolled,'” Adamic emails. “More than 57,000 people are registered [for his class], 12,000 watched all of week 1 lectures, and even fewer (6,000) submitted the first week’s assignment.”