Atmospheric pressure, how strong it is (in absolute terms, ~15 psi/100 kN/sq m), and how little we notice. A rough demonstration:

You know those 15-lb (~7 kg) dumbbells? Ever lifted one? They're heavy. (No bragging, please.)

Now, place your hand on a table, palm up, and rest the dumbbell on your palm. "Ow!", right? That hurts! It feels like your hand is getting crushed! Get that thing off!

Well, you've only increased the pressure that's normally on your hand by about 5%, plus or minus.