Rainbows are appearing on walls in historic Outer Banks lighthouse. What causes it?

When a 151-year-old lighthouse stands alongside the notorious “Graveyard of the Atlantic,” strange goings on are to be expected.

So it’s fitting that the National Park Service says brilliant colors have been found crawling across the walls inside the Bodie Island Lighthouse on North Carolina’s Outer Banks.

The rainbow-colored phenomenon appears at seemingly unpredictable intervals in the tower’s watch room, where they linger for a time, then vanish, according to Cape Hatteras National Seashore. The size varies, too, with the vibrant bands extending several feet on occasion.

It’s a bizarre light show that tourists don’t often see, park officials said, either because the lighthouse is not open or because tourists prefer to linger out on the tower’s balcony.

What they’re missing is more historic than supernatural and — increasingly rare.

The rainbows are caused by sunlight shining through the 344 glass prisms in the “first-order Fresnel lens” at the top of the lighthouse.

“When light enters the Bodie Island Lighthouse at the right angle, the 344 prisms ... can create some truly striking and vibrant rainbows,” Cape Hatteras National Seashore reports.

“These rainbows will show up anytime the sun is out and at the proper angle. ... We typically see the rainbows on the interior walls beneath where the Fresnel lens is located. However, the rainbows can be seen wherever the light has passed through the prism.”

The lights move across the walls as the sun moves, and linger as long as the sun is at the proper angle, the park says.

It’s a sight that can happen in any lighthouse still using its historic Fresnel lens, officials said. That means you won’t see it in the Cape Hatteras Lighthouse, which now has a modern rotary beacon.

The park’s 75-foot Ocracoke Lighthouse, built in 1823, is among the historic towers that still has an historic Fresnel lens, the park says.

North Carolina’s lighthouses were built to keep ships from running aground in an area known as the “Graveyard of the Atlantic.” It is believed as many as 2,000 shipwrecks are in the region, the North Carolina Maritime Museum reports.

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