The size and age clonal organisms can reach remains poorly known, with the largest natural clones so far detected extending over hundreds to thousands of meters and potentially centuries in age. The maximum clone age and size estimates reported in the literature are typically bound by the scale of sampling, and may grossly underestimate the maximum age and size of clonal organisms. The occurrence of clones of the slow-growing marine angiosperm Posidonia oceanica was examined at spatial scales ranging from meters to hundreds of kilometres using microsatellites on 1544 sampling units from a total of 40 locations across the Mediterranean Sea. This analysis revealed the presence, with a prevalence of 3.5 to 8.9 %, of very large clones spreading over one to several (up to 15) kilometres in different locations across the Mediterranean Sea. Using estimates and models of the clonal growth of P. oceanica, we estimated these large clones to span hundreds to thousands of years, suggesting the evolution of large phenotypic plasticity, general purpose genotypes in this species. These results, obtained combining genetics, demographic and model calculations, question our present knowledge and understanding of the spread capacity and lifespan of plant clones. These findings call for further research on this life history trait associated to clonality, considering its possible ecological and evolutionary implications.