The Vieques National Wildlife Refuge covers roughly half of the island — about 18,000 acres, the largest refuge in the Caribbean. Most of the famous beaches inside it (Red, Blue, Garcia) sit on former Navy bombing-range land that has been cleaned, surveyed, and reopened, while much of the eastern interior is still closed for ongoing remediation.
The refuge headquarters near the airport is the place to start: pick up a map, ask about closures, and sign in. Visitor hours are roughly 6 am to sunset. Stick to marked roads and beaches — the warning signs about unexploded ordnance are not for show.