Across:

1: H2O

3: Skillfully

4: The square root of four

5: Huge

9: A large kingdom

10: The opposite of subtract

11: A walk through the wilderness

12: Very good

15: A furry swimming mammal

18: The part of the body used

 for the auditory sense

19: Beings from outer space

21: A large deer like animal

22: He ___ to the park

23: The opposite of from