A thin layer of clay, just a few millimeters thick, marks a dramatic boundary in Earth's history. This layer separates the world of the dinosaurs from the one that followed their mass extinction. Recently, scientists discovered an unusually high concentration of a rare metal, iridium, within this compacted layer. Iridium is extremely rare on Earth’s surface but is common in meteorites. This finding challenged long-standing theories about how the dinosaurs disappeared so suddenly at the end of the Cretaceous period. The mass extinction that wiped out the dinosaurs has been known to scientists since the 19th century. For many years, the cause of this extinction remained a mystery. Most scientists believed it was caused by a catastrophic meteorite impact that darkened the sky and caused dramatic climate changes. However, this theory was not immediately accepted. The idea began with a physicist and his son, a geologist. They discovered that the clay layer contained 30 to 130 times more iridium than normal. This high concentration of a rare metal was found across the globe at the same geological layer, suggesting a global event rather than a local one. To understand the significance of this iridium layer, the team studied geological sections in Italy where Cretaceous and Tertiary rock layers meet. They tried to determine how long it took to deposit the clay by measuring the amount of cosmic dust, which falls continuously from space. Their initial hypothesis was that a higher concentration of iridium would mean a slower deposition. However, the results did not match their expectations. The layer was not the result of a slow accumulation of dust, but rather a sudden, massive input of material—something that could only have come from a cosmic impact. The discovery of this global iridium layer sparked extensive research and led to the identification of similar layers in many parts of the world, from Spain to Tunisia and across North America. This global pattern pointed to a single, catastrophic event. Iridium, a member of the platinum group, is rare on Earth because it tends to sink toward the planet’s core during its formation. Meteorites, by contrast, contain high levels of iridium because they have not undergone this internal differentiation. The only way to explain such a widespread and concentrated layer of iridium is a massive impact from an asteroid or comet. The proposed size of the impactor was a ten-kilometer-wide asteroid, large enough to scatter material across the globe. This theory, introduced in the early 1980s, was initially met with skepticism. Some scientists suggested alternative causes, such as volcanic activity or changes in sedimentation. However, the discovery of a massive crater off the coast of Mexico, measuring 180 kilometers in diameter, provided the missing evidence. This crater, formed by an asteroid impact 66 million years ago, matched the timing of the iridium layer and confirmed the impact theory. Despite this confirmation, some scientists continue to debate the exact sequence of events. Some argue that the impact may have occurred slightly before the extinction event, while others suggest that volcanic activity in what is now India could have also contributed to environmental stress. However, the majority of the scientific community agrees that the extinction of the dinosaurs was caused by a massive asteroid impact, an event that left its mark in a thin layer of clay that took over a century and a half to fully understand.