Ongoing Activity at Hunga Submarine Volcano, Tonga: The Case for Better Monitoring of Submarine Volcanoes Worldwide.

The powerful eruption of Hunga volcano (15‐January‐2022) excavated ∼6.3 km3 of pre‐existing material, leaving behind an 855 m deep crater. The scientific and humanitarian response to this event was challenging due to the remote location, safety concerns, and COVID‐19 pandemic restrictions. To investigate the status of ongoing eruptive/hydrothermal activity, this study used, for the first time, an un‐crewed surface vessel operated remotely from >16,000 km away to make direct water column measurements within the crater and map its structure in detail. Intense turbidity and oxidation‐reduction potential (ORP) anomalies located ongoing activity at sites on the steep inside crater slopes near both remaining islands. Mid‐water acoustic reflectors indicated ongoing degassing, and positive ORP anomalies suggested gas composition was dominated by CO2. At least 75% of the crater rim is shallower than 100 m, so any exchange with the surrounding ocean is limited by the depths of breaches in the rim (185 m between the islands and 290 m on the ENE side). This post‐eruption bathymetry results in accumulation of emission products within the deep crater. There were no indications of the ongoing activity visible at the ocean surface, which highlights the limitations and inherent biases associated with relying on discolored surface water and/or atmospheric disturbances to determine eruption start/end dates at submarine volcanoes. This study demonstrates the value and need to add repeat hydrothermal plume and bathymetric surveys to our toolbox for monitoring submarine volcanoes, and the potential for un‐crewed, remotely operated vessels to contribute significantly to these efforts. Plain Language Summary: The powerful eruption of Hunga volcano on 15‐January‐2022 sent a plume to space and generated unusual tsunamis. How the eruption impacted the submarine environment was more difficult to determine. We used a highly innovative, un‐crewed vessel equipped with instruments to directly measure characteristics of the water within the 850 m deep crater excavated during the eruption and map the crater's shape in detail. These measurements showed there was ongoing hydrothermal/volcanic activity and CO2 degassing within the crater 7 months after the eruption. The deepest parts of the crater are isolated from the surrounding ocean, so products of this activity become trapped within the crater. While the most powerful eruptions are rare, they can be quite hazardous. The results from this study emphasize the importance of monitoring submarine volcanoes long after signs of eruptions are no longer visible at the ocean surface or atmosphere. Operation of the un‐crewed vessel >16,000 km from the study site was a major technological achievement and the first time that scientists could monitor operations in real time from anywhere around the globe. The success of this mission demonstrated the potential of this innovative technology to contribute to broader applications in ocean exploration, monitoring, and event response. Key Points: Hydrothermal/volcanic activity and CO2 degassing continued at Hunga volcano 7 months after the explosive 15 January 2022 eruptionTurbidity and CO2 are accumulating within the deep crater, which is isolated from the surrounding ocean deeper than ∼200 mFirst use of a novel un‐crewed vessel to conduct over‐the‐horizon bathymetric and water column surveys operated >16,000 km from the study site [ABSTRACT FROM AUTHOR]