Mine have never set foot on outside ground! I've never let my cats outside. The only exception was Chaz, who was adopted from a fish store. He was used to going outside briefly and always wanted to wander. We tried our best to keep him inside, but he was very sneaky and got out several times.

The last time he was hit by a car and I found him under the hedges in the alley. I cried for days.

It's just too risky; cats are good at getting out of enclosed places, no matter how secure we think they are.