The footage was captured at the Glacier Bay National Park and Reserve. The park posted the video to its Facebook page and explained that the mirage is called a "Fata Morgana".
The strange dome-shaped object in the video is actually your brain being tricked into seeing a distorted version of an island.
A Fata Morgana is a mirage that appears just above the horizon and can significantly distort an object that's already there. Glacier Bay National Park and Reserve explained on Facebook: "Fata Morgana is a mirage seen within a narrow band on Earth’s horizon.
Islands in glacier bay turn to UFOs or the flying Dutchman with a little imagination and a pinch of cool science! When air of different densities meet, the air in Earth’s atmosphere acts as a refracting lens, creating the mirage effect we see.