It is the second busiest metro system in Europe, after the Moscow Metro, as well as the tenth busiest in the world. It carried 1.520 billion passengers in 2015, 4.16 million passengers a day, which amounts to 20% of the overall traffic in Paris. It is one of the densest metro systems in the world, with 244 stations within the of the City of Paris. Châtelet–Les Halles, with five Métro lines, three RER lines, is one of the world's largest metro stations. However, the system has generally poor accessibility, because most stations were built underground well before this became a consideration.