## 6 Conclusions  
This study established an evaluation system with terrain and geomorphological conditions engineering geological conditions, hydrogeological conditions, sensitive geological conditions, regional development levels, and ecological reserve constraints as guideline layers and utilized the AHP and the multi-objective linear weighting method to calculate the single-factor evaluation results. The principal findings of this study are summarized as follows:  
1. The evaluation indexes for the suitability of development and utilization of underground space in Nantong City can be divided into six categories and 15 indexes, among which the engineering geological conditions, hydrogeological conditions, regional development levels, and ecological reserves have a greater influence on the suitability of development, while the influence of topographic and geological conditions is relatively small. 2. Suitable, secondarily suitable, less suitable, and unsuitable development areas in Nantong City accounted for 14.74%, 30.66%, 37.17%, and 17.44% of the land area, respectively, with the suitable development areas mainly concentrated in Rugao City, Chongchuan District, and the southwest of Rudong County, etc. Problems existing in the development of underground space in the rest of the areas are mainly ground subsidence, soft soil, the liquefaction of sandy soil, and other undesirable engineering geologic problems. 3. Comparing the evaluation results with and without considering the regional development level and ecological reserve constraints, the results show that when considering the influence of regional development levels and ecological reserve constraints on underground space development, the suitable development area decreases significantly, and the secondarily suitable, less suitable, and unsuitable development areas increase, so the sustainable development factors have a greater influence on the suitability of underground space development. Therefore, the sustainable development factors have a greater impact on the suitability of underground space development, and it is necessary to consider the sustainable development factors when evaluating the suitability of underground space development. In summary, this research addresses the limitations of incomplete indicator selection in current research on the suitability of UUS development, providing a comprehensive evaluation system that considers the geological environment from a sustainable development perspective. It helps to make the development of UUS more in line with the current sustainable development strategy and maximizes the economic benefits generated by the development of underground space. The research results can provide a reliable theoretical basis for the planning of underground space in Nantong City, promoting the rational planning and sustainable development of underground space in Nantong City and other coastal cities. Conceptualization, P.Z. and T.J.; methodology, P.Z. and T.J.; software, T.J.; validation, [PERSON], [PERSON] and [PERSON]; formal analysis, [PERSON] and [PERSON]; investigation, [PERSON] and [PERSON]; resources, [PERSON]; data curation, [PERSON] and [PERSON]; writing--original draft preparation, [PERSON] and [PERSON]; writing--review and editing, [PERSON] and T.J.; visualization, [PERSON]; supervision, [PERSON]; project administration, [PERSON]; funding acquisition, [PERSON] All authors have read and agreed to the published version of the manuscript. **Funding:** This work was funded by the Natural Science Foundation of Hebei Province (Grant No. D2020207003); The Science Research Project of Hebei Education Department (Grant No. QN2022052); The Science Research and Development Program of Hebei University of Economics and Business (Grant No. 2023 QN16); The Hebei Province Higher Education Teaching Reform Research Project (Grant No. 2022 GJJG176). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.