This paper analyses the notion of Mediterranean 'Southern Thought' discussed by Italian philosopher Franco Cassano (1943-2021), through the lenses of critical geographical and geopolitical scholarship, drawing upon the concept of 'costal indentation' as addressed by one of its main interpreters, anarchist geographer Elisée Reclus (1830-1905). Southern Thought is increasingly associated with decolonial and post-development approaches, especially with notions such as degrowth, and the 'pluriverse'. It is first and foremost understood as an anti-dogmatic philosophy that opposes all fanaticisms and narrow (political, cultural or ethnic) chauvinisms, including the dogmas of development, market, speed and productivity coming from (neo)colonial 'Norths' to foster pluralism and mutual listening. Connecting and putting into dialogue different strands of scholarship in critical geopolitics and decoloniality, I first contend that Southern Thought can help enlarging geographical notions of plural Souths to avoid essentialising any single notion of 'Global South', or 'Southern Theory', which would imply the risk of reproducing new dogmatisms. Then, I argue that Southern Thought can engage productively with geography, and especially with scholarship in critical geopolitics addressing global crises such as the migrant and refugees one, in learning from these plural 'Souths' (including Southern Europe and the Mediterranean) to find alternative models to that of the nation-state based on territorial sovereignty and bound to territorial 'traps'.