How to Drill a Coral

For our research we are using corals as climate archives to look at environmental changes in the ocean. You can read more about the details here.

To get the corals there are two options. Either we take samples from living species underwater or we take samples from corals that were already taken out.

In 1967 the government of East Germany had the idea to recover an entire coral reef and ship it to Berlin as part of an exhibition in the Museum of Natural History. So, around ten men went to Cuba to take out 70 boxes filled with corals and send them home. 52 years later, some of those corals have never been truly unpacked, we decided, together with the Natural History Museum, to give those corals a second life and use them for our research. These corals can help us to gain a better understanding of how the ocean environment has developed before 1967. These results, together with new corals from Cuba, can help to reconstruct the development of the water temperature but also acidification of the surrounding waters. 

Under Water

The first step is to scout a coral – to find a so-called old stony coral. Stony corals build their massive skeleton from elements in the water column that they live in. Just like trees, they form layers every year that are visible in their skeleton and can be used to reconstruct the chemistry of the ocean throughout time. The coral that we look for needs to be old enough for us to get a nice long record from it. Once the perfect coral is found, the rest of the equipment and divers can be brought to the location.

The scientific divers use an underwater drill with a special drill head to drill into the coral. This drill can recover an entire core from the coral and not just drill a hole. We are not harming the coral because only the very top of the coral has living tissues while the rest inside is the skeleton made of calcium carbonate, like chalk. The process is very difficult and dangerous. The divers not only need to have an eye on their oxygen tank but have to resist the currents as well.

Drilling the coral and recovering the cores can take days. Each diver can only do 2 to 3 dives a day of around 1 hour. Sometimes pieces of the equipment fall off and sink down so they have to be searched for and recovered. The drill can also get stuck and has to be freed with a hammer before the drilling can continue.

Once the drill reaches its maximum depth the divers can take out the coral core to be able to continue drilling the next sections. The final core will be in several parts and will be pieced together on land like a puzzle.


Once the divers are done drilling, the hole is filled with underwater epoxy cement, with similar properties as the coral itself. This helps avoid burrowing animals from entering the hole and the coral tissues will grow back over it.

The round shapes on top of the coral are the individual coral polyps. Together they form a colony. Only the living surface of the coral is covered by polyps, everything below that is the skeleton. During summer months with warmer temperatures and more sunlight, the corals are happier and grow faster so the skeleton is less dense, while in winter, the growth slows down and the skeleton is denser. X-ray images reveal this growth pattern as annual density banding pairs. When drilling into the coral, we recover a core as seen in the graphic below, this is then used to reconstruct temperature, salinity and pH of the seawater. We can see changes not only from year to year but sometimes monthly going back up to 500 years.

On Land

First we went to Berlin to get some corals from the Museum for Naturkunde

The tools we used: a drill …

The core won’t come out easily

But finally… 

… and water, to keep the coral cool.