The synchronous fireflies–fireflies that blink in unison–of Great Smoky Mountains National Park have become such a huge attraction that the park limits access to watch their two-week-long show every June at Elkmont. Visitors must enter a lottery to get a peek; this year’s lottery opens Friday, April 27, at noon. You’ll need good luck: Last year, 18,000 people applied for 1,800 passes.