The though of coral reefs conjures up visions of a wide variety of colourful organisms living in clear, warm and shallow waters; but this is only the « tip of an iceberg », mainly from the surface to 30m depth. Compared to shallow reefs, what is below 30m, the so-called Mesophotic Coral Ecosystems (MCEs), characterized by the presence of light-dependant corals, remain quite a mystery. The legal and logistical constrains to access the MCEs are the most important hurdles that researchers are currently facing, which explained that 2/3 of MCEs remains understudied and their poor state of conservation. MCE and shallow reefs share some similarities, playing a key role in ecosystem services and goods, and being build around a keystone species: corals. The main difference is that we know far more on shallow reefs, than we do on MCEs. However the last years scientists and managers began to realize the importance of MCEs due to the fact that on coral reefs, the impacts of human and natural perturbations typically diminish with depth and distance from shore. It has been hypothesized that deep reefs serve as refuge for shallow reef habitats, but only tested in a few locations for few species. Today corals face growing risks in the Anthropocene epoch and will likely be the first victims of accelerating pace of environmental change, causing local to regional extinctions. So it is timely to expand coral research to these yet unexplored depths to unravel unknown biodiversity and identify areas that may be key players in the resilience of decimated shallow reefs. Depth is a critical challenge for research in MCEs. So to open up unexplored MCEs to scientists in the Pacific, DEEPHOPE will benefit from a unique partnership with “Under the Pole Expedition” (UTP) team, that will provide the cutting edge diving technology (with experimented deep divers), the availability of remotely operated vehicles and access to the boat “Why” to explore French Polynesia MCEs for 10 months. Around this partnership and to bring to lights the existence of MCEs, we build an interdisciplinary and international consortium of 12 scientists from 4 countries with strongly transverse competences (microbiology, physiology & ecology as well as acidification, sounds, and oceanography) to combine their efforts and address several critical goals relevant to MCEs: (1) Identify MCEs in French Polynesia and assess their coral diversity and abundance; (2) Evaluate the contribution of MCEs to the replenishment of threatened coral populations in shallow reefs; and (3) get new insights into the coral adaptation and/or acclimatization to deep environments. Despite MCE coordinated programmes were initiated the last few years in some countries; presently, France lags well behind the above mentioned teams in terms of studies dealing with MCEs (with only two papers published on MCEs from French overseas territory), despite France ranks third on a world-wide basis in terms of total surface area of its coral reefs. Hence, France has a responsibility to act for the sustainable conservation of MCEs and DEEPHOPE is the only-of-its kind scientific project dedicated to the study of MCEs in one of the French overseas territories. People care for what they know and the MCE is one of Earth’s last unknown prolific habitat types, thus hitherto condemned to society disregard. By exploring an under-exploited ecological areas to this day, we will fill in important scientific knowledge gaps and respond to a big societal challenge defined by ANR, which is how to sustainably manage the resources and adapt to climate change. Enhancing protection and resilience of coral reefs on Earth is a major challenge of our time for all societies all over the world and DEEPHOPE represents a unique opportunity to undertake cutting-edge research on MCEs with global significance, that will hopefully, convince of the needs to protect reefs as a whole, and not only the first 30m.