Reuters

College debt. Law school debt. Medical school debt. These are all topics that have been hashed over ad nauseam in the past few years.

But when was the last time you heard anything about Ph.D. debt?

Unless you're an academic, my guess is you haven't. Doctoral programs still have a reputation for giving their students a (mostly) free ride by providing living stipends and teaching opportunities along with tuition breaks. And most of the time, the rep holds true.

But not always. In some cases, Ph.D.'s leave school with the same kind of mountainous loan bills we've come to expect for pre-professional students. This week, Karen Kelsky, a former tenured anthropology professor turned consultant and blogger, kicked off a conversation about this dirty little secret of the ivory tower by asking Ph.D.'s to report the debt they stacked up in grad school on a public Google doc and explain how they accumulated it. Hundreds responded—enough that the sheet temporarily appeared to crash from the traffic.

Ph.D. Debt broke Google. — Karen Kelsky (@ProfessorIsIn) January 16, 2014

Thankfully, we don't need to rely solely on crowdsourcing to get a sense of how much debt Ph.D.'s take on during their studies. According to the National Science Foundation's annual (and comprehensive) Survey of Earned Doctorates, about 63 percent of Ph.D.'s completed their degrees in 2012 with no grad school-related debt. However, about 18 percent bookwormed their way to more than $30,000 in loans.

There were also vast variations between disciplines. More than three quarters of engineers graduated debt-free, and only 7.7 percent broke the $30,000 mark. In comparison, just under half of humanities Ph.D.'s finished without the help of student loans, and 28 percent needed more than $30,000 worth of them to support themselves. In 2012, 312 humanities students graduated more than $90,000 in the hole.