# Additive Manufacturing in Underwater Applications
## 1 Introduction  
Additive manufacturing (AM), commonly named 3D printing, is a promising technology with many applications, including usage in the aerospace industry [1, 2, 3], biomedical applications [4, 5], the construction industry [6, 7], and many others [8, 9, 10]. One particular application is 4D-printed advanced solutions [11, 12, 13]. In the case of producing a physical object using 4D printing additive technology, it should be understood as a group of technical solutions where the layers of material respond to physical factors as well as other multi-materials with various properties, especially in time. As a result of manufacturing in 4D printing technology, materials often change shape, size, or pattern depending on factors such as the temperature, moisture, and stress relaxation in the material [11, 12, 13]. AM technology has a number of benefits, one of them being automatization and the possibility to work in harsh environments, including in space [14, 15] or underwater [16].  
Using AM technologies for underwater applications brings a number of advantages--for instance, increasing the safety of the work, limiting the environmental burden, and high efficiency [16, 17]. The design and development of AM technologies for underwater applications require a different approach than in the case of surface technologies. Insome ways, this new environment can support the process, such as with better cooling properties, but, in many cases, it brings more challenges than in 3D printing in the air [18; 19]. Additionally, the design of modern filaments for effective application in a harsh water environment requires a complex approach [20]. Currently, only a few prototype applications for this technology have been developed as real elements eligible for underwater 3D printing [21; 22]. Most of the ideas presented as \"underwater 3D printing\" are simply technologies using AM as a tool for the manufacturing of elements for further underwater application [16; 23]. [PERSON] et al. [24] mention 3D printing technology as one of the most important for the marine industry as a whole. The authors pay attention to a small number of studies provided in this area and show this direction as one of the most prospective [24]. However, underwater AM is a promising means to develop new, effective applications on a larger scale. The technology itself, as well as the materials used, still require development and optimization.  
The main focus of this study lies in the current development of 3D printing technology for underwater applications, as well as underwater applications involving part fabrication via AM. In particular, the article describes the technologies based on extrusion for concrete and the laser deposition molding of titanium powder as technologies tested in the laboratory for underwater 3D printing. Some contemporary applications for this technology have been tested in underwater environments, but there are still a number of problems to be solved. The improvements in the technology itself and new materials are discussed. The article is addressed to researchers and practitioners, to demonstrate the state of the art in the field of underwater 3D printing and to help them to obtain useful information to conduct research in this field. It also shows the most promising directions for further development in this area.