Roman concrete could repair its own cracks. Some Roman buildings have stood for 2,000 years, while modern concrete can start to crumble within decades. For a long time, scientists did not fully understand why. The Romans made their concrete from lime, water and volcanic ash from the area around Pozzuoli, near Naples. The ash reacted with the lime and created a material that grew stronger over time instead of weaker. In 2023, a team from MIT led by Admir Masic found another piece of the puzzle, small white lumps of lime scattered through the concrete. Scientists had always thought these lumps were a sign of careless mixing. In fact, they were the repair kit. When a crack forms and rainwater gets in, the lime dissolves and refills the crack with new mineral, sealing it from the inside. The secret was “hot mixing.” Instead of using slaked lime, the Romans added quicklime directly to the mix. It heated up as it reacted with water and left these lumps behind on purpose. To test the idea, the team cracked samples of concrete made with the Roman method and let water run through them. Within two weeks, the cracks had sealed completely. In concrete made without the lime lumps, the water kept flowing. Roman harbour concrete went even further. Pliny the Elder wrote that once it touched the waves, it became a single block of stone that grew stronger every day. In 2017, geologist Marie Jackson showed he was right, seawater slowly grows rare crystals inside the concrete that hold it together.