The Great Lakes are still largely iced over after a long, cold winter. The frozen cover is melting at a slower rate than ever before recorded. On April 22, NASA recorded data showing 33.9 percent of the waterways were still covered by ice. During a normal year, at that date only 2 percent of the Great Lakes are still covered in ice. 

Ice on Lake Superior was so thick during the winter that tourists were able to safely travel over the surface to visit ice formations in the Apostle Island caves. The water body is still 63 percent covered in ice, averaging nine inches thick. 

"This has been a remarkable ice cave season, the longest and best one anyone can remember," Bob Krumenaker, Apostle Islands National Lakeshore superintendent, said. 

The Aqua satellite captured images of the lakes still largely frozen a full month after the beginning of astronomical spring. That observatory was launched in 2002, on a mission to study the water cycle of the Earth. The mission has already exceeded its projected 10-year mission life. 

"[A]s many as 70 ships have been backed up in Lakes Michigan, Huron, and Erie, waiting for passage into ports on Lake Superior. The U.S. Coast Guard has been grouping ships together into small convoys after they pass through locks at Sault Ste. Marie, in order to maximize ice-breaking efficiency and to protect ships from damage," NASA reported in a statement on the persistent ice cover. 

In 1979, a record 94.7 percent of the Great Lakes were encased in ice. The least amount of frozen cover occurred in 2002, when only 9.5 percent of water on the Great Lakes was covered in ice. 

During the winter of 2013-2014, Lake Superior reached its greatest ice coverage on March 19, when 95.3 percent of the lake was frozen over. That water body is the largest freshwater lake in the world in terms of area, and third by volume. 

Many ducks have died of starvation in the Great Lakes area, unable to hunt for fish beneath the frozen surface. Spawning programs aimed at restocking species of fish will need to be delayed by program managers, waiting for warmer waters. 

Since 1979, winters with greater-than average ice cover on the Great Lakes were followed by cool summers. This could indicate a mild summer, but many factors will affect average temperatures. 

Despite these frigid coverings, average water temperatures on the lakes has been rising over the last 30 to 40 years, according to NASA.