Many of America’s national parks remained open during the country’s longest-ever government shutdown. But few had enough National Park Service employees working to protect the areas and keep watch of visitors. Those visitors did a lot of damage in 35 days. They walked over sensitive lands. They opened gates while no one was watching. One person even struck one of California’s famous Joshua Trees with a vehicle. When park service employees returned to work last week, they began cleaning up and repairing what they could. But environmental experts warn that the damage in some places could take years to recover. Even before the shutdown, national parks faced $12 billion in much-needed repair work. That number has only grown. Many parks had no employees working during the shutdown. Others had skeleton crews.