In February 1970, with the school's storied quadrangle by the Charles River still in the grip of winter, Harvard University broke the bad news to students and their parents: Tuition was going up.

Their reluctant consensus raised the annual cost of attending the prestigious school in the fall of 1971 by $200—to $2,600. It was the first time since 1949 that the school, which was chartered in 1650, had boosted tuition two years in a row.

"It used to be that once in an undergraduate career tuition would increase," acting dean John T. Dunlop told The Crimson, the student newspaper. "But from now on, unless inflation is halted, there's no choice in the matter but to continue raising tuition."

More than forty years later, tuition at American colleges and universities continues to surge ahead—much faster than the inflation Dunlop cited. This fall, Harvard's annual tuition and fees (not including room and board) will set you back $45,278, more than 17 times the 1971-72 cost. If annual increases had simply tracked the inflation rate since 1971, next year's tuition would be to just $15,189.

It's not just the tuition costs at elite universities like Harvard that are outpacing the government's Consumer Price Index.

The average cost of tuition and fees at a private, non-profit, four-year university this school year was $31,231—up sharply from $1,832 in 1971-1972 (in current dollars). At public, four-year schools, tuition and fees cost about $9,139 this year. In the 1971 school year, they added up to less than $500 in current dollars, according to the College Board.

"If you look at the long-term trend, [college tuition] has been rising almost six percent above the rate of inflation," said Ray Franke, a professor of education at the University of Massachusetts, Boston. "That's brought immense pressure from the media and general public, asking whether college is still worth it."

Despite the annual sticker shock, millions of American students and their families still believe it is—and many experts concur, pointing out that college graduates, on average, do still make considerably more than those with just a high school diploma. Since 1971, annual college enrollment has more than doubled in the U.S. to 19.5 million, as of 2013, the latest Census data available. In that year, there were 5.3 million in two-year colleges, 10.5 million in four-year colleges and 3.7 million in graduate school.

But after decades of expanding enrollments, applications have begun tapering off. College enrollment peaked in 2011. With millions of recent debt-burdened college graduates still underemployed by one of the worst recessions on record, many students and their families are wondering why tuition keeps rising, and how much higher it will go.

Meanwhile, the ongoing rise in costs have stretched families' capacity to pay for college to nearly the breaking point.

In 1971, for example, Harvard's $2,600 tuition amounted to about 13 weeks' worth of the median household's annual income of $10,285. Today, the median household needs to work for almost a year to pay the full sticker price.

To make up the gap, millions of students and families every year are forced deeper into debt to make up the difference—around $100 billion a year is borrowed through a cottage industry of private and publicly-funded loan programs.

With the total level of student debt outstanding at more than $1.2 trillion, parents, students and researchers are now asking: how is this sustainable? And why does college cost so much?