Last week, when the polar vortex forced all but the hardiest Minnesotans to huddle indoors, a Duluth utility crew labored outside in a 50-below wind chill, sledgehammering through 5 feet of frozen soil beneath a neighborhood street.

The night before, a 6-inch water main buried beneath the street had broken, sending water spewing. It quickly turned to treacherous ice.

"This section has been problematic for us," said city utilities supervisor Chris Kleist. "We've had a number of breaks here over the last few years."