1) Robert Downey Jr. is an American actor known for his portrayal of Iron Man in the Marvel Cinematic Universe. He has also starred in films such as "Sherlock Holmes," "Chaplin," and "Tropic Thunder," and has received critical acclaim and commercial success for his performances.

2) The Great Barrier Reef in Australia is the largest coral reef system in the world and is visible from space.

3) In Norse mythology, the World Tree, known as Yggdrasil, is a massive ash tree that connects the nine realms of existence. It is considered the center of the cosmos and serves as a conduit between different realms, including Asgard (realm of the gods), Midgard (realm of humans), and Helheim (realm of the dead).

4) Hugh Jackman is an Australian actor, singer, and producer best known for his portrayal of Wolverine in the "X-Men" film series. He has also starred in musicals such as "Les Misérables" and "The Greatest Showman" and has received critical acclaim for his performances.