Zac Efron Shirtless & Showing off his Calvin's at the Beach

Over the weekend Zac Efron spent the day at the beach celebrating his HSM co-star Ashley Tisdale's birthday. The 23-year-old actor spent most of the day shirtless and showing off his Calvin Klein underwear, which at one point turned inside out and revealed his crack!

15 pictures of Zac Efron looking sexy as always inside!