UnScramble this Word: i s e r s t g?

Questions

What do you call water that moves beneath the earth's surface?

Who starred with Fred Astaire in the Christmas movie "Holiday Inn"?

Who played two or more roles in The Dark Mirror?

At which lengthy battle did 10,000 Athenians defeat 20,000 Persians?

What country issued a platinum coin in 1988 bearing a koala?