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At the end of the last ice age, billions of boulders and cobbles were left scattered over southern Scandinavia. Huge ice sheets had dragged the rocks from mountainous regions further north down to the Baltic Sea areas. When those ice sheets retreated and sea levels rose, these colossal boulders became rocky reefs, rich with marine life. These rocks formed part of a naturally resilient coast, proving a rugged structure for waves to break and reducing pressure on the seashore.

For centuries these reefs have been exploited as building materials. This practice of "stonefishing" occurred extensively across Denmark and was only banned in 2010 by which time the rocky shore had been depleted, leaving only smaller rocks on the seabed. Now that it's illegal, Denmark is scrambling to recover marine life and prepare for rising sea levels by restoring these reefs.

In the century between 1900 and 2000, approximately 8.3 million cubic metres of rock covering an area of about 21 square miles were extracted from Danish shallow coastal waters for building purposes on land. Although this is a relatively small area, boulder reefs are biological hotspots, supporting hundreds of marine species. These rocky reefs act as a base for everything from oysters to seaweed to thrive, plus a safe haven for young fish.

Read more: How Denmark's oysters are transforming foodies into citizen scientists

Now numerous projects are being carried out to bring the stones back to coastal areas. At least eight stone reef restoration projects are currently underway, with the earliest having begun even before stonefishing was prohibited. The first project happened in northern Denmark, conveniently close to Norway where a quarry provided the needed rocks. Since then, interest in restoring lost reefs has grown tremendously.

The restoration projects reveal that marine life returns when given the time, space and right conditions to do so. Seaweed forests recover, creating necessary structural complexity and associated species reenter the restored areas. Atlantic cod, once a culturally and economically important species in the region, are especially attracted to the restored reefs. One study showed cod numbers increased by 60-129 fold over the course of the five to six months after a rock reef was rebuilt.

While working on a stone reef restoration project on the island of Als, in southern Denmark, one of us (Jon C. Svendsen) spotted an opportunity. With rising sea levels, weather getting more severe and being a low-lying country, Denmark is becoming increasingly concerned with coastal erosion. Rocky reefs can help dissipate wave energy, providing a form of protection and so it just made sense to combine the two.

Although Denmark is not as low lying as the nearby Netherlands, the coastline is 4,600 miles long. This makes it one of the longest coastlines in Europe, with around 40% of the population living within a couple of miles of the sea. As sea levels rise, larger waves will be able to reach the shore, leading to increased risk of flooding and erosion, especially on coasts with very small variation in tide levels such as Denmark.

Our stone barrier reef project tests the idea of combining both the protection of the coastline and enhance biodiversity along a section of coast on the island of Sams. The boulder reef was constructed earlier this year, and extends about 100m long and 16m wide. Sitting roughly 1m below the sea surface, it resembles a medieval rock wall rising from the seabed and runs parallel to the shore.

This reef design is expected to partially shelter the coastline from waves, causing large waves to break along its crest. This reduces the amount of energy available for coastal erosion and encourages the build up of sediment. It is hoped that within this newly created sheltered area, meadows of seagrass (a marine flowering plant) can colonise and flourish.

We'll be monitoring the progress through an extensive monitoring programme, recording changes in species richness and the sea bed. By snorkelling, we can survey the changes in marine plant and animal life. Underwater cameras will be used to unobtrusively identify and count the number of species moving around the reef over the next three years. With evidence of how marine life recovers, we'll explore whether these rocky reefs can jointly stabilise the coastline and improve biodiversity in the area.

Globally the fundamental problem with coastal management is that we cannot see vast changes as they occur below the sea surface. Various initiatives across the UK are also trying to make amends for the large historic loss of marine habitats.

Oyster reefs are one such example. Following the collapse of the oyster industry in the 1980s, there are now more than a dozen restoration projects underway in a bid to restore populations of these ecologically and environmentally important species. By working with nature, not extracting from it, we finally stand a chance of building truly resilient coastlines.

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