Humanity risks a sixth mass extinction due to accelerating biodiversity loss. Despite scientific consensus about severity and scale of human-driven species extirpation, and calls for transformative action, public understanding remains largely unstudied across diverse global populations. A representative survey of 2,808 participants across five countries on five continents (Brazil, India, South Africa, United Kingdom, United States) assessed awareness, beliefs, and behavioural responses to biodiversity loss and mass extinction. Findings reveal a striking awareness-acceptance paradox: only 23% had heard the term "sixth mass extinction," yet 86% agreed it may be happening once the concept was explained. Participants correctly identified human activity as the primary driver (85%) and expected severe consequences for climate, health, and food security. Most supported the need for transformative changes (79% for strengthening protections) and expressed willingness to adopt conservation policies and behaviours, especially for low-cost options (75-97%). However, participants perceived widespread scientific disagreement about whether mass extinction was human-caused (92%) and nearly half incorrectly perceived it was reversible (48%). Belief in proximate human causes, perceived impacts and risk, trust in science and personal experiences of nature loss predicted support for transformative change. Participants in Brazil, India, and South Africa expressed higher concern compared to those in the UK and US, challenging assumptions about environmental engagement in developed versus developing countries. These findings have critical implications for science communication and conservation policy, revealing untapped public readiness for action while highlighting the urgent need for targeted awareness campaigns that address the awareness-acceptance gap and leverage existing experiences and concern to drive transformative change