## 1 Introduction  
Artificial reefs are man-made structures that are intentionally submerged to mimic some of the features of natural reefs [1, 2]. The first traces of artificial habitats in aquatic environments date back to the Neolithic period (9000-3000 BC), when coastal vegetation and rocks were intentionally submerged to create new fishing grounds off the coast of Africa [3]. Their use was also a common practice in traditional fishing communities, especially in Southeast Asia [4]. The main goal was to increase catches by creating additional structures that provided protection for fish. Research into the role of artificial underwater habitats in attracting, concentrating, and catching fish has been conducted since the 1930s [5, 6]. However, the effectiveness of artificial reefs in this regard is controversial. On the one hand, there are studies showing the effectiveness of this method [5, 7], but others negate the effectiveness of using artificial reefs for this purpose [8]. Currently, this kind of solution obtains a wider meaning; the main aim of the application of artificial reefs is usually ecosystem restoration, particularly the improvement of biodiversity and support of the reproduction and habitation of underwater life [9; 10; 11].  
3D printing technology brings a lot of benefits, including freedom of design--creating complex shapes without formwork, materials efficiency, and the possibility of working in harsh environments. In this article, the terms additive manufacturing and 3D printing, despite existing differences, are used as synonyms. This review is focused on the practical aspects of this technology. Currently, additive technology finds many applications in different areas, such as medicine, aerospace, automotive, and construction industries, and others [12; 13; 14; 15; 16]. Also, in recent years, this technology has been employed for ecological projects, especially those connected with artificial reefs. The usage of additive manufacturing for this purpose gives researchers the possibility to design surfaces with greater complexity and roughness, which is important for the settlement of water organisms, such as corals or mussels and other marine life [17; 18]. Some research shows that this technology can effectively support ecological design and engineering, especially in restoration efforts [19; 20]. It allows for the matching of artificial reef structures to specific requirements of local ecosystems [21]. Moreover, additive manufacturing can effectively support the shift toward ecological sustainability [19; 22]. This technology is also scalable and cost-efficient [23].  
The main aim of this article is to demonstrate the state of the art and further perspectives for the development of artificial coral reefs. This article is based on a critical analysis of the literature, supported by selected case studies. This article describes current technologies used in the creation of artificial reefs, putting emphasis on additive manufacturing, evaluates currently used materials, and summarizes the influence of this technology on ecosystems through the analysis of selected case studies. It also discusses the challenges and limitations of current technologies used in 3D printing artificial reefs as well as presents current trends and further directions. The main audience of this work is researchers and practitioners, and the presented knowledge can help them to acquire useful information to develop the research connected with artificial reefs.