More than 100 million years ago, vascular plants inhabiting the intertidal zone adapted to live in the sea, giving rise to seagrasses. The first seagrass fossils (Posidonia) date back to the Cretaceous, around 120 million years ago (Blondel 2010). Seagrasses are key species found in shallow waters around the world (Orth et al. 2006) down to 90 m depth at maximum (Duarte 1991), and the majority of seagrass ecosystems grow in sheltered coastal environments where coastal communities have primarily settled over time. Already by the end of the 20th century, about 40% of the human population inhabited the coastal zone (Independent World Commission on the Oceans 1998) and the trend is increasing (Neumann et al. 2015), providing evidence of the potential interactions between human activities and seagrass meadows through time. Humans spread through the world from Africa about 60 000 years ago, using the coastal zone as a corridor to reach Australia, and later on followed the coastline once again to colonize America (Stringer 2000; Oppenheimer 2009). The artefacts left behind by these coastal communities were flooded following the gradual 120 m sea-level rise occurring over the last 20 000 years (Lambeck and Chappell 2001) and subsequently covered by sediments allowing the growth of seagrass meadows that overgrew and protected this heritage. While many of the coastal areas that hosted early human settlements are now located at water depths too deep for modern seagrass meadows to thrive, it is likely that past meadows growing in those areas as well as the deepest-growing extant meadows, may have played a role in the initial burial of these sites. The archaeological artefacts embedded within sedimentary layers below seagrass meadows range from ships (wrecks) to prehistoric fishing and other flint tools, textiles, weapons and ceramics ((Fischer 2011; Abelli et al. 2016); Table S1). Such items have been discovered when excavating ancient coastal plains subsequently flooded and covered by seagrass (Fischer 2011; Soter and Katsonopoulou 2011) or when the artefacts became exposed following seagrass loss and sediment erosion (Fischer 2011; Gregory and Manders 2016).

Due to their combined high productivity, capacity to attenuate waves and currents and to trap and bind particles, seagrass meadows raise the seafloor (Duarte et al. 2013). A recent survey reported an average difference in short-term sediment elevation rates between seagrass-vegetated and unvegetated areas of 31 mm per year with large variability between meadows (Potouroglou et al. 2017). The persistence of seagrass rhizomes, roots and leaf sheaths through time, due to the anoxic conditions prevailing in these deposits and the recalcitrant nature of seagrass remains, leads to the formation of sediment deposits of varying thickness with long-term sediment accumulation rates (SAR) ranging from 0.6 to 5 mm year−1 (Marbà et al. 2015; Serrano et al. 2016a), keeping in mind that the SAR in surface sediments may be overestimated due to biomixing especially in non-Posidonia meadows (Johannessen and Macdonald 2016). While seagrass meadows in general have the potential to stabilize sediments, protect underlying archaeological layers and serve as historical archives, the thick seagrass deposits may in addition embed archaeological artefacts.

The capacity of seagrass meadows to bury and preserve archaeological artefacts is influenced by interactions of biological factors such as growth pattern, meadow productivity, cover and density, chemical factors such as recalcitrance of seagrass debris and physical factors such as water depth, hydrodynamic energy and soil accumulation rates (Serrano et al. 2016b). Large and long-living seagrass meadows of the genera Posidonia and Thalassia can build organic-rich deposits several meters in thickness in certain habitats (Mateo et al. 1997; Lo Iocano et al. 2008; Duarte et al. 2013), while opportunistic and/or low biomass seagrass meadows of the genera Halophila and Zostera do not build similarly thick sediments. Seagrass meadows inhabiting areas with e.g. low hydrodynamic activity, fairly rapid sediment deposition and high sedimentary organic carbon content with low oxygen concentrations should be seen as a suitable habitat for preservation of archaeological heritage. In highly depositional environments, even meadows formed by small and fast-growing species, can exhibit enhanced capacity for sediment accumulation (Potouroglou et al. 2017). Hence, linking aspects of seagrass habitat, physical aspects of the environment and seagrass life history provides a context for understanding their potential role in preserving archaeological remains.

Permanency of seagrass deposits is obviously a key requirement for the protection of archaeological remains by seagrasses, and the many case studies reported below document situations where this requirement has been fulfilled. However, various climate- and human-induced environmental processes have been impacting seagrass during the Late Holocene, and the study of Posidonia mats in the NW Mediterranean Sea revealed effects of factors such as enhanced continental soil erosion and eutrophication of coastal waters since Roman-Medieval times (López-Merino et al. 2017) even though losses of seagrasses have only been reported since the 20th century. Major losses have occurred due to events such as the wasting disease, which extirpated most of the north-Atlantic eelgrass populations in the 1930s (e.g. Rasmussen 1973) and worldwide mainly due to human impacts accelerating in the late 20th century (Orth et al. 2006; Waycott et al. 2009). Such losses have led to exposure of archaeological remains (Fischer 2011) and major changes in the seafloor, especially in exposed settings even though roots and rhizomes may still exert a stabilizing effect years after seagrass decline (Rasmussen 1973). A recent study also demonstrated that seagrass loss triggers the erosion of historic carbon deposits while revegetation effectively restores seagrass carbon sequestration capacity (Marbà et al. 2015).

The age of sedimentary deposits under extant seagrass meadows can be up to 6000 years (Lo Iocano et al. 2008). These deposits are now receiving significant attention because of the large organic carbon stocks contained therein, ranging between 4.2 and 8.4 Pg of organic carbon within the top meter of seagrass soils worldwide (Fourqurean et al. 2012). However, the role of seagrass deposits in preserving underwater archaeological heritage (Fischer 2011; Polzer 2012) and recording human development through time remains unaccounted for in assessments of the cultural services provided by seagrasses despite the link between seagrass ecology and marine archaeology being implicitly made already in 1969 when seagrass debris was successfully used to determine the period when a ship sunk in Malta (Frost 1969). This shipwreck was buried below a 4-m-thick P. oceanica mat, and was estimated to have been buried 1100 cal. year BP, as identified, probably for the first time, by radiocarbon dating of the seagrass mat (Frost 1969), yielding the earliest estimate of seagrass sediment accretion rate of about 4 mm year1. However, once removed by excavation, seagrasses are often not capable to re-establish leading to the exposure of the artefacts, compromising their preservation (Godfrey et al. 2005).