126. These measures include, above all, an obligation of each State to make a serious and complete assessment of the repercussions on the environment before executing any project that could harm the environment of third States. 127. In this regard, Principle 17 of the Rio Declaration on the Environment and Development adopted by the 1992 United Nations Conference on the Environment and Development, establishes that: “Environmental impact assessment, as a national instrument, shall be undertaken for proposed activities that are likely to have a significant adverse impact on the environment and are subject to a decision of a competent national authority.” 74 128. This principle is now considered part of general international law, as recognized by the International Court of Justice in the case of Pulp Mills on the River Uruguay: “It may now be considered a requirement under general international law to undertake an environmental impact assessment where there is a risk that the proposed industrial activity may have a significant adverse impact in a transboundary context.” 75 129. The International Court of Justice defined and confirmed the existence and the content of this obligation in its decision of December 16, 2015, in the joindered cases of Certain activities carried out by Nicaragua in the border area (Costa Rica v. Nicaragua) and Construction of a road in Costa Rica along the San Juan River (Nicaragua v. Costa Rica). Citing an extract from the case of Pulp Mills on the River Uruguay mentioned in the preceding paragraph, that Court added: “Although the Court's statement in the Pulp Mills case refers to industrial activities, the underlying principle applies generally to proposed activities which may have a significant adverse impact in a transboundary context. Thus, to fulfil its obligation to exercise due diligence in preventing significant transboundary environmental harm, a State must, before embarking on an activity having the potential adversely to affect the environment of another State, ascertain if there is a risk of significant transboundary harm, which would trigger the requirement to carry out an environmental impact assessment.” 76 The International Court of Justice also asserted that: “A State's obligation to exercise due diligence in preventing significant transboundary harm requires that State to ascertain whether there is a risk of significant transboundary harm prior to undertaking an activity having the potential adversely to affect the environment of another State. If that is the case, the State concerned must conduct an environmental impact assessment. The obligation in question rests on the State pursuing 74 The Rio Declaration on the Environment and Development is available at: http://www.unep.org/documents.multilingual/default.asp?documentid=78&articleid=1163. 75 I.C.J., Case concerning Pulp Mills on the River Uruguay (Argentina contra Uruguay), judgment of April 10, 2010, p. 73, para. 205 76 I.C.J., Cases concerning Certain activities carried out by Nicaragua in the border area (Costa Rica v. Nicaragua) and Construction of a road in Costa Rica along the San Juan River (Nicaragua v. Costa Rica), judgment of December 16, 2015, p. 45, para. 104. 60