Final answer:
The Cave of Lascaux was closed to the public in 1963 because visitors contributed to the growth of algae and a calcite veil on the cave walls, which threatened the preservation of the cave art.
Step-by-step explanation:
The Cave of Lascaux, discovered in 1940, was closed to the public in 1963 due to the threats posed by the influx of visitors to the preservation of the cave's prehistoric paintings. The visitors' presence led to the development of algae and a calcite veil on the walls, alongside the erosion of the cave floor. To protect the site and preserve it for future study, authorities decided to restrict access to the original caves and, in response, created a replica, Lascaux II, which simulates the look, smell, and atmosphere of the original. The Cave of Lascaux is now a UNESCO World Heritage Site, which requires ongoing monitoring and maintenance to safeguard its historical significance.