**1. Introduction**

Cities are important compositions of human civilization and, arguably, the main space formations of human society. The study of cities, therefore, is of value to society, not only concerning contemporary issues but also in relation to how they developed previously. This is especially the case for cities, or rather urban sites, which have been impacted by historical natural disasters such as earthquakes, tsunamis, and floods that resulted in destruction or the disruption of urban civilization. The work of excavating and researching such urban sites through archaeological methods has, therefore, become a hot topic in academia [1–5]. Well known urban archaeological sites including Pompeii, Volubilis, and the archaeological ensemble of Tarraco, have become important assets of international cultural heritage, and have received much attention in western countries. Among them, Pompeii was built

in the 6th century BC but was subsequently destroyed as a result of the Mount Vesuvius eruption in 79 AD, where streets and houses were buried by volcanic ash. Remarkably, these houses have been preserved relatively completely, and archaeological excavations have continued since 1748 AD, providing important information enhancing our understanding of social life, culture and the art of ancient Rome [6,7]. Consequently, the city of Pompeii has been designated as a world cultural and natural heritage site by the United Nations Educational, Scientific and Cultural Organization (UNESCO).

In China, there are also many unique urban areas, similar to "Pompeii", forming what is known as a "city overlap city" landscape because in different historical periods, cities have been built successively in the same area under the influence of natural and social factors, and there is an obvious vertical overlapping phenomenon. Several cities including Kaifeng, Shangqiu, Xuzhou, and Xiapi, which are located in the Yellow River floodplain, characteristically exhibit this special urban landscape phenomenon which was essentially produced through flooding of the Yellow River in the historical period [8–11]. Among these cities, Kaifeng city, the only provincial city in the Yellow River floodplain, has suffered the most from Yellow River floods; for instance, it has been destroyed several times throughout history. Although these cities and their relationship to the Yellow River can be considered to be highly relevant in research on the impact of environmental change, especially concerning human coping mechanisms, research in this area has hitherto been limited to mere "landscape descriptions" and, therefore, little is known about the region's stratigraphic characteristics. This limitation is generally regarded as being related to the fact that the floodplain in the lower reaches of the Yellow River is in an economically underdeveloped region, and consequently, the government, as well as most other sectors of society, is not sufficiently aware of the natural and human heritage intrinsic to these unique cities and the region.

Therefore, this study takes Kaifeng as a case in point, using archaeological data and historical geographical analysis to study the form of surface remains. Primary data were collected from four drill cores which were obtained from different locations at Kaifeng city. The sedimentary cycles were quantitatively divided-out and dated, and the form of Kaifeng's underground city heritage was thus revealed, at least, from the perspective of stratigraphy. The purpose of this study is not only to explain the shape of this "city overlap city" landscape as a harmonious production, made by human beings and nature in the context of the Yellow River floods, but also, in terms of a representation of the "wisdom of the ancients" which has an important heritage value. It is also intended that this study can be used as a reference for other such studies in the Yellow River floodplain.
