In December 2016, after it was announced that Phase 2 might cost $6 billion, transit experts expressed concern that the Second Avenue Subway might be so excessively costly as to preclude construction of Phases 3 and 4, as well as future expansions. One expert stated that the Phase 1 project was the most expensive subway project in the world, and that compared to other subway systems around the world, the cost of building new subways in New York City was much higher. The Second Avenue Subway's per-mile construction cost is higher than that of other projects in similar cities like London's Crossrail and Paris's Grand Paris Express, which themselves are among the most expensive underground-railway projects in the world. MTA officials stated that the Second Avenue Subway cost as much as it did only because of the complex underground infrastructure in Manhattan, as well as the fact that the New York City Subway operates 24/7 service.