The fish clay at Stevns Klint is enriched in iridium, an element rare in the Earth's crust and comparatively common in asteroids. That enrichment, found here and at a handful of other sites, is the physical evidence behind the Alvarez hypothesis: that the mass extinction which closed the Cretaceous roughly sixty-six million years ago was triggered by an asteroid strike, the crater of which lies buried beneath Mexico's Yucatan Peninsula. Stevns Klint is one of the best-exposed records of that boundary anywhere on Earth, which is why UNESCO inscribed it as a World Heritage Site on 23 June 2014 under criterion viii, the geological criterion, on the same day as the Wadden Sea. You can walk along the shore and put your hand flat against the layer. It is a strange thing to touch a bad afternoon.

Below the boundary lies the Tor Formation, uppermost Maastrichtian chalk laid down between roughly seventy-two and sixty-six million years ago in a relatively deep, quiet sea. More than 450 species of macrofossil have been counted in that Cretaceous chalk, and hundreds more microfossils. Above the fish clay comes ten to thirty centimetres of darker clay and chalk representing a sea floor of miserably low diversity, dominated by the pioneer bivalve Corbulamella - the ecological equivalent of weeds colonising a burnt field. Then, slowly, the bryozoan chalk of the Rødvig and Stevns Klint Formations builds back up through the early Danian. Over 830 species of macrofauna have been identified from the cliff in total. Stevns Klint was also the first site anywhere to show ammonites surviving briefly into the Paleogene, after everyone had assumed they died at the line.

On the clifftop stands Højerup gl. Kirke, a small limestone church that has been slowly losing its argument with the sea. Folklore gave it an origin story: a skipper caught in a storm off the reef vowed that if he survived he would build a church on the cliff, and built it, unwisely, right at the edge. J. L. Heiberg put the legend into his play Elverhøj in 1828, including the detail that the church saves itself by shuffling inland the width of a cock's stride every Christmas night. It did not shuffle fast enough. On 16 March 1928 the chancel and a section of the churchyard went over the edge and down onto the beach. The nave still stands, propped and braced, with a viewing balcony where the altar used to be. A replacement church had already been built a few hundred metres back from the drop.

The bryozoan chalk that makes the cliff is unusually resistant to shock, and in the early 1950s the Danish military took an interest in that fact. Stevnsfortet was cut into the rock between Rødvig and Højerup, running some 1.6 kilometres of tunnel eighteen to twenty metres below the surface, with living quarters, command rooms, a hospital and a chapel. Its armament was salvaged history: two armoured turrets carrying two 150 mm guns apiece, secondary battery pieces from the German battleship Gneisenau that the occupying forces had already emplaced once, on Fanø. They could reach twenty-three kilometres, covering the southern approach to the Øresund and the invasion beaches of Køge and Faxe bays. Paired with the fort on Langeland, the job was to keep the Warsaw Pact's Baltic fleet bottled up. The command was stood down in 2000; the tunnels reopened in 2008 as a Cold War museum. Down there, the K-Pg boundary runs along the wall.

One story on this coast is about an impact that ended three-quarters of species on Earth and left a smear of iridium behind. The other is about a generation of Danish conscripts sitting inside that same rock, waiting for an exchange that would have done something comparable and never came. The fort likes to be associated with the Cuban missile crisis, and one persistent claim has intelligence on the Soviet missile shipments coming from Stevns - but those ships went through the Great Belt, not the Øresund, and the sightings came from Hammerodde lighthouse, the Gedser Rev lightship and Langelandsfortet. Above ground the cliff simply carries on eroding, seventeen kilometres of white wall up to forty metres high, dropping fresh chalk and fresh fossils onto the beach every winter.