Colombia

Off Cartagena in Colombia, an underwater wall has been catching sand for the city for almost 250 years.

In 1640, Portuguese ships were wrecked in the widest entrance to Cartagena’s bay. Sand gathered around the wrecks until the entrance silted up and an island had grown together with the peninsula.

Ships had to take a narrower way in, and the city’s defences moved there.

A hundred years later, the sea began to reopen the passage. To keep enemies out, the Spanish built a wall under the water, almost two kilometres long, from wooden piles and stone.

The wall is still there. Ships cannot pass it, but sand gets caught against it. The beach has grown outwards, and the narrow peninsula where high-rises and hotels stand today has doubled in size.