Blockchain for unmanned underwater drones: Research issues, challenges, trends and future directions

Two-thirds of the earth's surface is surrounded by water and the majority of it is still unexplored. The underwater monitoring of the oceans and their surroundings is highly crucial from several perspectives, e.g., to unearth the hidden minerals/oils, to monitor the life of underwater species, military and rescue applications, surveillance of maritime borders, and predict tidal wave behaviors among others. The exploration and inspection activities of the underwater environment are mainly performed remotely by unmanned underwater vehicles or robots because human direct involvement in such a complex environment might not be a feasible option. Moreover, the recent maturity in digital automation and underwater drone technology along with the emergence and improvements in wireless, communication, and sensing technologies had already encouraged the research community to drive deep into this domain. Among several disruptive enabling technologies, Blockchain has emerged as a vital enabling technology to fulfil key requirements for secure data sharing, storage, process tracking, collaboration, and resource management. This study presents a comprehensive review of the utilization of Blockchain in different underwater applications, discussing various use cases along with detailing blockchainbased architectures, potential challenges, solutions, and future research directions. Potential challenges of underwater applications addressed by Blockchain have been detailed. This work identifies knowledge gaps between theoretical research and real-time Blockchain integration in realistic underwater drone applications. The key limitations for effective integration of Blockchain in real-time integration in Unmanned Underwater Drones (UUD) applications, along with directions for future research have been presented.