Deep-down definition

I've kept my identity hidden for five years now, but I knew deep down inside that it couldn't last forever.

I guess deep down I trusted them.

Pumps, however, may be (and have been) placed deep down in boreholes, so that water may be pumped from much greater depths.

Quinn feels that way too and they know deep down it's time to go into hiding.

Sure, we can be practical and deep down we know it's all smoke but—God, it's fun to dream!