Millions and thousands of years ago, a warm sea covered the territory of what is now the Nizhny Novgorod region. Giant prehistoric creatures and mammoths roamed here, and the climate varied from very warm, tropical conditions to harsh continental ones. Remains of these giants have been discovered by both paleontologists and casual observers, right near the Nizhny Novgorod Kremlin. The ancient walls of the Kremlin preserve fragments of fossilized sea lilies and sea urchin spines. The remains of fossilized organisms, with histories spanning millions of years, are surprisingly woven into the landscape of Nizhny Novgorod. They have been found in the Nizhny Novgorod Kremlin, on the slopes of the Oka and Volga rivers, on the present-day Piskunova Street, and even in the marble used at the Gorkovskaya metro station. These remarkable finds are presented to locals and tourists alike in a panel exhibition titled “Traces of Vanished Worlds” near the Nikolskaya Tower of the Kremlin. During the Carboniferous Period, about 300 million years ago, this region was covered by a warm sea with numerous small and large islands. The climate was tropical. On land, 30-meter-tall lepidodendrons, 20-meter-tall horsetails, and tree-like ferns flourished. During that period, a huge number of invertebrates inhabited the "Nizhny Novgorod Sea." These invertebrates actively absorbed calcium from the water to build their skeletons and shells. After these organisms died, their skeletons and shells formed massive layers of limestone. In the Nizhny Novgorod region, these deposits exist but lie too deep underground. The limestone with traces of Carboniferous invertebrates used to clad the towers of the Nizhny Novgorod Kremlin during its restoration for the city's 800th anniversary was brought in from the Vladimir region. The Vladimir limestone contained many skeletons of solitary corals, snail shells, and sea urchin spines. These spines are clearly visible in the cladding of the Kremlin’s towers and walls. The spines of prehistoric sea urchins can be seen in the cladding of the Nikolskaya Tower of the Nizhny Novgorod Kremlin. If you look closely at the walls of the Arsenal and the Zachatyevskaya Tower, you can see the elongated shells of fusulinids – an extinct genus of protozoans from the Carboniferous Period. These shells are shaped like grains and are aligned along their axis of coiling. Guests from the distant past are also found in the elements of urban landscape design – for example, in the picturesque limestone boulders on the Kremlin’s lawns. This limestone, which is native to Nizhny Novgorod, formed later – in the seas of the Permian Period. After the Carboniferous Period came the Permian. During this time, around 265 million years ago, the sea began to gradually recede, and ancient amphibians appeared in the region, resembling modern reptiles. Among them were Uralerpeton and Yarilinus. Fragments of these extinct reptiles were found on the slopes of the Oka River near Romodanovsky station, and Yarilinus was even named after the place of its discovery – the Yaril ravine. Scutosaur remains – parareptiles from the late Permian period, whose name means “shield-toothed lizards” – were also found in this area. These lizards reached a length of up to 3.5 meters, and their bodies were covered with large armor plates. Despite their fearsome appearance, scutosaurs were herbivores that fed on soft vegetation, similar to modern hippos. Scutosaurs were hunted by theriodonts of the genus Inostrancevia – the first large land predators with fangs up to 15 cm long. Remains of these predators have also been found in the Nizhny Novgorod area. Meanwhile, echoes of the Paleogene period of the Cenozoic era can be found in the walls of the Nizhpoligraf building, lined with nummulitic limestone brought from the Crimean Peninsula. Nummulites (from Latin, meaning "coin") are an extinct genus of single-celled organisms. They lived in flat, highly coiled shells. During the Anthropogene Period of the Cenozoic era, during the largest Dnieper glaciation, the territory of the Nizhny Novgorod region was covered by a glacier with a thickness of up to two kilometers! During interglacial periods, the glacier retreated, but later the last glaciation began – the Valdai Glaciation, about 110,000 years ago. At that time, woolly mammoths and rhinoceroses, primitive bison, giant deer, and muskoxen appeared in the area of the future Nizhny Novgorod region. The bones of these animals can be found throughout the region. Local woolly mammoths were up to 3.5 meters tall and weighed about 7.5 tons. Their enormous bodies were covered in fur up to one and a half meters long. Mammoths lived for an average of 50 years. They migrated across the modern Nizhny Novgorod region in groups of 20-30 individuals, led by an experienced female. Typically, the group consisted of her sisters and daughters with their young. Adult males joined the herd only in the summer. Mammoth remains have been found over the years during construction work on Zelensky and Pokhvalinsky descents in Nizhny Novgorod. It is clear that mammoths roamed where people later built the Kremlin. One of the most recent finds is a fragment of a mammoth femur discovered in 2023 near the village of Novinki. Interestingly, mammoths are not the ancestors of elephants. They appeared later and became extinct without leaving any direct descendants. The lands of present-day Nizhny Novgorod during the Anthropogene period were also traversed by woolly rhinoceroses. These animals stood up to two meters tall and weighed about two tons. Their barrel-shaped bodies were covered with long, light-brown fur. In Nizhny Novgorod, the teeth and cervical vertebra of woolly rhinoceroses have been found. The teeth were hidden in the soil layers at Zelensky descent, while the cervical vertebra was found by children during a walk along the Volga River near the Chkalov staircase.