1.
underground
2.
interchange
3.
intramural
4.
submarine
5.
underneath
6.
midsummer
7.
interstate
8.
supersonic
9.
undercover
10. intrastate
11. interact
12. midway
13. underpass
14. superstar
15. subway
16. intersect
17. supervise
18. interface
19. subtotal
20. interview