This chapter finds that the ISA defines ‘marine environment’ broadly and imposes a threshold requirement – ‘serious harm’ – to the definition of ‘marine environmental damage’ in DSM. It argues that the determination of whether the threshold has been crossed should be based on both factual assessment and value judgment. The measure of marine environmental damage in DSM is a much more complex issue. This chapter draws on experience in other contexts, including the International Oil Pollution Compensation Funds, the ‘F4’ claims before the United Nations Compensation Commission, the US and EU laws and practice and discussions in the context of the Convention on Biological Diversity. Upon examination, it proposes to rely primarily on a cost-based method, while valuation of pure environmental damage or interim loss secondary. It suggests a two-step pattern for measuring environmental damage in DSM.

As a preliminary issue, this chapter argues that the DSM regime embodies a dual-track regulatory system where national regulation by the sponsoring State plays a subordinate role complementary to international regulation of the ISA. Thereafter, it examines a network of international environmental obligations of the sponsoring State under the overarching principle of sustainable development, in accordance with both lex specialis of DSM and general international environmental law, drawing on the Seabed Disputes Chamber’s Advisory Opinion of 2011. That covers the sponsoring State’s obligation of prevention, obligation to apply the precautionary approach and obligation concerning environmental impact assessment (EIA). Then, it turns to the contractor, investigating its international environmental obligations to conduct EIAs and submit EISs, to prepare for and respond to environmental emergency, on environmental management and monitoring and to apply the highest environmental standards. The latter part refers frequently to the 2019 draft Exploitation Regulations and the still to be adopted Phase 1 Standards and Guidelines of the LTC.