Exploring mathematical connections with relevant current issues is an important support for learning in a world where reality and its problems are becoming increasingly global, multidimensional and complex. The first author of this paper has developed, from scratch, a set of 17 worksheets relating mathematics with the sea, driven by the cause of ocean sustainability. The tasks are intended for middle school mathematics, grades 7 to 9, and address all the main topics of the syllabus (algebra, geometry, statistics, computational thinking). Most of the tasks are contextualised in the history and livelihood of Cascais, the Portuguese municipality where the author teaches, focusing on aspects such as marine pollution, fishing, oceanographic exploration, surfing, navigation, lighthouses, among others. During the academic year 2022-23, a didactic experiment was carried out, by assigning some of the tasks to middle school students, collecting data and conducting a survey afterwards. From the results of the didactic experiment, the following aspects stand out: the established connections enabled the students to recognise the relevance of mathematics, thus leading them to understand it is part of reality and not merely an isolated school subject; the students got involved with curiosity and commitment in carrying out the tasks; they detected incorrect resolution strategies and self-corrected them; they developed transversal mathematical skills and deepened other areas of knowledge, going beyond learning specific mathematical contents. The tasks provided the students moments for reflection and the sharing of experiences of active citizenship in favour of the ocean, proving that mathematics classes can contribute to raising awareness of the importance of ocean sustainability and benefit from this connection. This work culminates with the challenge of putting into practice or adapting these tasks in other Portuguese schools and municipalities, thus reinforcing the students' learning process in a country with a maritime dimension such as Portugal.

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