Extreme-depth water-related optical imaging: conquering ultra-low illumination environments from epipelagic zone to Mariana Trench

Abstract Exploring the ocean’s vast, water-related environment, covering over 70% of Earth’s surface, remains a formidable challenge due to photon starvation, high-pressure extremes, and complex light-scattering effects below the photic zone. Optical imaging technologies have emerged as transformative tools for full ocean depth exploration, overcoming limitations of traditional acoustic methods through high-resolution, spectrally rich, and temporally precise observations. This review systematically surveys the physical principles, engineering constraints, and state-of-the-art developments in optical imaging from surface waters to the Mariana Trench. We analyze the role of blue-green pulsed lasers in improving imaging quality. We highlight key factors affecting light propagation in seawater. Advanced imaging modalities such as polarized imaging, range-gated imaging, single-photon imaging, streak camera techniques, and ghost imaging (GI) are examined for their capabilities to enhance visibility, resolution, and resilience in turbid, light-limited conditions. Furthermore, we introduce the progress achieved by deep-sea submersibles and their high-performance camera payloads is highlighted, alongside the burgeoning integration of artificial-intelligence-driven image enhancement and restoration frameworks. Collectively, these interdisciplinary innovations chart a new path for unlocking deep-sea frontiers, enabling ecological monitoring, resource mapping, and autonomous guidance in earth’s most inaccessible water-related realms.