Kolindsund (English: Sound of Kolind) is a 26.4-square-kilometer (10.2 sq mi)[1] elongated lakebed on the Djursland peninsula in Denmark.

[4][5] During the Stone Age, Kolindsund was a saltwater sound that separated the Djursland peninsula from the mainland, creating an island.

By the Middle Ages, the entrance of the sound had been blocked off near the town of Grenaa, turning the area into a navigable lake, which likely only had a depth of a few meters.

[2][better source needed] Throughout all of the sound there is a system of drainage pipes which divert water towards the middle canals and then into pumping stations.

The fertile clay-like soil in the sound consists mainly of two types of aquatic deposits, marine and freshwater silt.

[4] Today, there is concern that possible future rising sea levels due to climate change might make it more difficult to keep the former lake dry as farmland.

In the past few decades, the region has seen an increase in flooding, which has led to frequent crop loss in the neighboring areas not protected by the Kolindsund's pumping system.

When the pumps started draining the lake in May 1874, the water level reportedly sank 1.3 centimeters (0.51 in) per day, eventually creating 2,800 ha of new farmland.

In reality groundwater from the surrounding hills has a tendency to run under the cut-off canals and into the former lake, often surfacing as wellsprings that formed bogs, causing problems for farming.

As a result, large parts of the sound were marshy fields that were too wet for plowing and growing of more intensive grain-based crops, and only suitable for grazing in summer.

The high yields of Kolindsund are in part based on the nutrient-rich silt-based soil, deposited through millennia at the bottom of the former lake and sound in layers that are up to 20 meters thick.

In some places the fields are colored with white streaks of prehistoric banks of oyster and mussel shell that have been ploughed up, giving witness to the marine past of Kolindsund.