Seafloor massive sulfides (SMS) occur at the depth of 1,000 to 3,500 meters. Because the areal extent of hydrothermal vents is relatively localized, mining at any one site leaves a footprint of only 10 to 20 hectares, and the mining will only last from one to two years. But multiple sites are likely to be developed over a longer period of time as part of a single mining lease, as was the case for the Solwara project (AMC 2018).

Abyssal plains lie 4,000 to 6,500 meters beneath the ocean surface. In the prototypic mining operation evaluated in the EU BCA, the designated site would be mined annually for 20 years at the rate of 135 km<sup>2</sup> per year. The total footprint over the operational life of the mine would be about 2,705 km<sup>2</sup> . This site lies in the national waters of the Cook Islands about 300 km from the nearest island.

Ecological characteristics differ within and across the three types of formations, and the associated ecosystem processes have different spatial and temporal scales (Dunn et al., 2018). Exploratory activities have turned up rich biodiversity in all of these areas, including abyssal plains which feature low biological productivity (Le et al., 2017). Sediment compaction and habit removal associated with DSM will destroy ecological communities in the mining footprint, and sediment plumes, which may contain toxics, can travel beyond the footprints (Washburn et al., 2019). Computer simulation of plume behavior is at the early stages (Heffernan, 2019). Light, noise, and sonic vibrations will be produced by DSM equipment, and sonic vibrations can occur from operating the riser lift system. Handling return water presents technical and cost issues; releases may occur near the seabed, or take place at mid-water (Heffernan, 2019). Tailings plumes from dewatering operations contain fine particles under 10 μm, and may drift beyond mining sites (Washburn et al., 2019). Of particular concern to marine scientist is the cumulative effects of these impacts on ecosystem functioning and connectivity as multiple seabed mining sites are developed over time (Dunn et al., 2018).

The existing information about DSM impacts on the environment is poor. Acquiring information on deep seabed environments is costly and technically challenging. Exploratory leases have been used to complement other studies on marine ecosystems. Expert surveys have also been used to help assess the environmental effects (Clark et al., 2016; Washburn et al., 2019). In a recent survey of expert opinion, the authors state that a “.. striking result is ... the relative paucity of experts willing and able . . . to offer an opinion about the vulnerability of mineral-associated habitats to a variety of risk sources ...” (Washburn et al., 2019, pp 36). The authors suggest that “survey outcomes . . . underscore the need for risk assessment to progress from expert opinion with low certainty to data-rich and ecosystem-relevant scientific research assessments to yield much higher certainty” (Washburn et al., 2019, pp 25).<sup>8</sup>

> 8 The environmental impact of deep-sea mining might be weighed against terrestrial mining of low grade deposits of the same metals, an activity that disturbs habits, and produces waste slags and toxic leachates that can damage ground water and terrestrial ecosystems. However, more is known about the environmental effects of terrestrial mining than the effects of seabed mining. The lack of knowledge about marine biodiversity and ecosystem function, and the possible consequences of disrupting dynamically complex seabed ecosystems with little information about the effects, would suggest a cautious, precautionary stance is warranted in the case of seabed mining.

5