Worldwide, there is a large amount of untapped geothermal resources located either offshore or onshore under lakes. To investigate the possibility of exploiting these geothermal resources, exploration techniques need to be adapted to certain requirements. This is because the same methods used on land simply do not work underwater. During exploration, the focus is on finding fluids and fluid pathways. Fluid pathways or structural features such as faults, pockmarks, hot springs, or fluid seeps, can usually be located underwater by indirect methods, such as bathymetric measurements or seismic surveys. However, detailed mapping and sampling of the seafloor or lake bottom requires more refined technologies. For our studies, we built an underwater field laboratory that combines structural and geochemical methods to explore underwater geothermal resources. We use bathymetry to locate fluid pathways and a remotely operated underwater vehicle (ROV) equipped with an underwater GPS system for mapping and sampling points of interest. The ROV can collect samples for fluid and sediment analyses using a fluid or sediment sampler mounted on the drone. It records physicochemical parameters such as salinity and water temperature, while also taking high-resolution videos and photos. This makes otherwise inaccessible or hard-to-reach areas visible. The use of rugged materials in manufacturing the ROV allows it to operate in extreme environments with higher temperatures and low pH levels. Red laser beams at the front of the ROV enable underwater distance measurements to estimate the size of e.g., springs or fractures. In 2023, three field campaigns were conducted at three lakes with different scientific objectives to test and evaluate the functionality of this unique lab for underwater exploration. We describe the different lakes (Lake Zurich and Lake Cadagno, Switzerland and Lake Ranau, Indonesia) in terms of their geological context and lake characteristics, the workflow adapted to the research question, to finally summarize the limitations and opportunities of ROVs in geothermal exploration campaigns.

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