Undersea cables, which carry nearly all intercontinental data traffic, are increasingly being used as scientific tools. These cables span over 1.5 million kilometers globally, laid on the ocean floor at depths of up to 8000-9000 meters. Since 2015-2016, scientists have used them to detect seismic waves with a sensitivity comparable to that of traditional seismological stations. This technique, known as "distributed acoustic sensing" (DAS), involves sending laser pulses through the fiber optic cables. As the light travels, it reflects off tiny imperfections in the fiber, creating measurable echoes. Seismic or acoustic waves cause deformations in the fiber, altering these reflections and allowing scientists to detect changes in the environment. Anthony Sladen, a research scientist at CNRS and affiliated with the Géoazur laboratory, tested this method in 2018 on a 41.5 km undersea cable off Toulon, France, successfully recording an earthquake of magnitude 1.9. Diane Rivet, an adjunct physicist at the Côte d'Azur observatory, is leading the Abyss project, funded by the European Research Council, to monitor the Chilean subduction zone using a 400 km undersea cable with over 30,000 detection points. The project began in 2023 and will run until October 2027. Unlike Japan, which has its own seismic and tsunami monitoring systems, Chile lacks a real-time monitoring network. Data from DAS could help develop a new early warning system for earthquakes and tsunamis in the region. DAS is also being used to study other phenomena, such as storm waves before they hit the coast, whale vocalizations, changes in seawater temperature, landslides, and underwater noise from maritime traffic. However, analyzing the massive data sets generated by DAS is complex. In 2020, researchers from the Norwegian University of Science and Technology (NTNU) collected 250 terabytes of data over 40 days from 120 km of fiber, capturing more than 800 whale calls. Researchers like Léa Bouffaut at Cornell University are developing artificial intelligence to detect marine mammal vocalizations in these data sets. DAS has limitations, as the laser light loses energy over distances beyond 150 km. Using underwater repeaters, similar to those used in telecommunications, can extend the measurement range over thousands of kilometers. Hybrid cables, designed to transmit digital data and collect scientific information, are now being developed. One such cable, TamTam, is expected to be operational by the end of 2027 and will cross the New Hebrides Trench, one of the world's most active subduction zones. Four observation nodes equipped with various sensors will be installed along a 411 km cable. Fabrice Cotton, scientific coordinator of the Smart cable SAFAtor project, emphasizes that data from these cables must not threaten the security of the countries through which they pass. Martin Patriat, a geologist at Ifremer and also involved in the project, notes that this is the first time such technology is being used on a telecommunications undersea cable. An autonomous submarine will place a traditional sensor next to the nodes to visually inspect the installation and validate the collected data. John Wrottesley, general director of the International Cable Protection Committee, points out that the United Nations Convention on the Law of the Sea (UNCLOS), which has been in force since 1994, defines the rights and responsibilities of states regarding maritime spaces, including undersea cables. However, UNCLOS does not specifically define marine scientific research, leaving its interpretation to the discretion of coastal states. Some countries impose few requirements for research, while others may even classify accidental events, such as the drift of an Argo buoy, as scientific research, requiring consent. Camino Kavanagh, a researcher at the United Nations Institute for Disarmament Research, notes that during World War I, cutting German undersea telegraph cables was a central objective of British military strategy. Kristi Govella, a lecturer in Japanese politics and international relations at the University of Oxford, adds that today, cabling technologies are increasingly politicized and secured due to geopolitical competition and the importance of data and digital sovereignty. Scientific research using these cables intersects with these geopolitical concerns. Anthony Sladen highlights that scientists must demonstrate the potential of these technologies for science and society to gain access to telecom infrastructure from operators, governments, or tech companies.