El Capitolio Nacional is Havana's most ambitious and grandiose building. It was constructed between 1926 and 1929 by 5,000 workers at a cost of US$17 million. While similar to the Capitol in Washington DC, it was actually modeled on the Panthéon in Paris. Set in the floor directly below the dome is a copy of a 24-carat diamond from which highway distances between Havana and all sites in Cuba are calculated. The building served as the seat of the legislature until 1959, and after eight years of renovation, was reopened to the public in March 2018. Guided tours are mandatory and last about one hour.