Hong Kong SAR, China, is home to more coral species than the Caribbean but some coral populations dropped by up to 51% between 2011 and 2019 due to pollution and coral mining. Now a start-up called ArchiREEF hopes to reverse the damage. Its 3D-printed tiles from terracotta clay can act as artificial coral reefs, as they mimic the hexagonal shape of a local species called brain coral. ArchiREEF has already laid 130 such artificial reefs on the seabed off Hong Kong’s coastline. So while coral reefs are the planet's most threatened ecosystems, here's how these artificial reefs could help save them.