Video (00:33) : Instagram user Nina Hale captured this video of skaters taking to the ice on Lake of the Isles on Dec. 10.

One moment, Jason and Holly Funk were zooming across Little Sand Lake on their all-terrain vehicle to do some ice fishing near their Park Rapids, Minn., home.

In the next moment, their four-wheeler broke through the ice and the couple plunged into the frigid water. Jason went under twice before linking arms with Holly, who clung to a piece of ice as they both grew weaker by the minute.

Hubbard County deputies and a citizen arrived 25 minutes later to rescue Jason, 46, and Holly, 41. Both were treated at a hospital for hypothermia. They were released this week. “It’s unfathomable we survived. God was holding us there,” Holly said. “It was so scary.”

Their ordeal has law enforcement and the Department of Natural Resources warning that ice conditions across the state are unstable and unpredictable, and to the naked eye, there is no way to tell the strength of ice by its appearance, the temperature, or whether or not the ice is covered with snow.

The DNR had only one lake reported as totally ice covered as of Thursday: Lake Dodo, a 76-acre lake north of Duluth.

The DNR says ice thickness must reach 4 inches before it is safe for walking or ice fishing, 5-7 inches to support a snowmobile or all-terrain vehicle, 8-12 inches for a car or small pickup and 12-15 for a medium-size truck.

The best way to tell if the ice is safe is to drill a hole with an auger or chip with an ice pick, then use a measuring tape to get an accurate depth. It’s best to check every 150 feet as depths can change from point to point, said Lisa Dugan, the DNR’s recreation safety outreach coordinator.

Since most of us don’t carry around those tools, Dugan said at minimum to ask people coming off the lake. They might know where trouble spots are.

Dugan takes ice safety a step further: wear your life jacket. She called them essential for winter activities on frozen water.

“The DNR preaches to wear life jackets if you are on a boat or beach or on the water at any time in the summer,” she said. “It is equally if not more important in cold water as it is the one piece of equipment that can keep your head above water should you fall in.”

Holly Funk was not wearing one when she plunged into the icy water Sunday. She may next time.

“That would have been huge,” she said. “We need to rethink our gear.”

The Funks have routinely ice fished Little Sand Lake by Dec. 10 for the past nine years without incident, but this year ice conditions are running a week to 10 days behind schedule, said Pete Boulay with the Minnesota Climatology Office.

The DNR defines “ice in” as when a lake is covered with ice, but depth is not factored in.

In the Park Rapids area, the median ice-in date for most lakes is mid- to late November. In the metro area, that usually occurs before Dec. 5, with larger lakes such as Minnetonka a few days after that. In 2002, Lake Minnetonka wasn’t declared frozen until Jan. 2.

After a cold start to November that allowed small and medium-sized lakes to freeze over early, a balmy end of the month that brought record highs caused them to thaw. It’s only been in the past week that ice has re-formed on many of those lakes, and it’s not thick enough for most activities.

The ice was only 2 inches thick where the Funks went through, the Hubbard County Sheriff’s Office said.

“People think that because we have flipped the calendar to December and ice was thick last year and the year before that, that it’s OK,” Dugan said. “It’s not the same year after year.”

The year has already produced three ice-related fatalities compared with two last winter and none in 2016.

“Just because there is ice cover, that does not make it safe,” Boulay said. “Even in the dead of winter, conditions can be very difficult to determine.”

Added Hannah Mishler, DNR conservation officer: “Ice is never 100 percent safe regardless of where you are. It can be really deceptive; you can’t see anything under the snow so you may not see cracks or dangerous areas. Safety is the most important thing out on the ice.”