The Inner Harbor is more than 90% frozen, according to the U.S. National Ice Center.

Plastic bottles, food wrappers and trash are stuck in the frosty waters. Small waves froze while hitting the motionless Coast Guard ship and museum on Pier 5. A rock from a nearby playground that someone threw to test if it would break the ice sat in the middle of the harbor.

It’s a rare sight in the coldest winter in Maryland in years. The last time the region saw comparable ice buildup was likely in 2018, said Andrew Snyder, a meteorologist with the National Weather Service. Long stretches of cold air, along with nighttime periods that are below freezing, will cause ice to remain frozen or build up, he said.

Despite the frozen conditions, public marine terminals are operating normally, according to Richard Scher, director of communications at Maryland Port Administration.

The Potomac River and the northern portion of the Chesapeake Bay are also more than 90% frozen, according to the U.S. National Ice Center.

A lobe of the polar vortex, a ring of cold air that typically circles the arctic, hit much of the United States this week, lowering temperatures to single digits that caused schools to close.

Snyder warned people not to venture onto ice-covered water. It takes at least 4 inches of ice thickness to support a human, he said, and “it’s really not worth risking it.” Hypothermia would set in very quickly, because water temperatures are around 33 degrees.

Howard County authorities reminded parents to talk to kids about ice safety, adding that “falling through a frozen pond, lake or river can kill you.”

Firefighters responded to reports of a dog who fell through a frozen pond in Ellicott City earlier in the week. They rescued the dog and returned him to his family.

This month, a pickup truck plunged from the Arlington Memorial Bridge into the icy Potomac River. The District of Columbia’s fire and emergency medical services department used airboats that move over ice on the river to reach the driver. The victim died after spending roughly an hour in the water, authorities said.