In 1969, Portugal and South Africa signed an agreement that enabled the apartheid regime effectively to extend its tentacles of empire over the proposed dam at Cahora Bassa. Located in the heart of central Mozambique, far from the South African frontier, the dam became a security project masked as a development initiative. Cahora Bassa, including the massive lake behind the dam, rather than becoming the multi-purpose dam originally envisaged by Portuguese planners, was transformed into a strategic security project designed to blunt the advance into southern Mozambique of the liberation movement the Frente de Libertacão de Mocambique (Frelimo) and, by extension, its ally the African National Congress (ANC). In this way, the colonial regime and its apartheid ally hoped to prevent attacks against Mozambique’s major colonial urban areas of Beira and Lourenco Marques. From Pretoria’s perspective, it was preferable to fight the ANC in central Mozambique rather than along the Limpopo river – southern Mozambique’s frontier with South Africa. The hydro-electric project at Cahora Bassa also ensured South Africa’s energy security. The 1969 agreement guaranteed that 82 per cent of the electricity generated there would be exported to South Africa at well below the world price. This article examines the vital role played by South African interests in financing, constructing, and defending the dam site at Songo, which resulted in the dam’s deterritorialisation and allowed South Africa to incorporate Cahora Bassa as an outpost of empire. It also explores the ways in which Pretoria held the dam hostage after Mozambique became independent, as part of a broader process of destabilising its socialist neighbour to the north. Even after the dismantling of the apartheid regime in 1994, the new ANC-led government resisted efforts to abrogate the colonial agreement, and Songo remained a foreign enclave. Although Mozambique regained sovereignty over the dam site in 2007, even today, most of the electricity generated by Cahora Bassa is exported to South Africa, at a price that remains a state secret.