Phrases of surprised reaction to news of what humans are doing, at least in my own mind, are trite. Mainly because I use one of the following phrases all too frequently ruminating on the latest bit of news I've encountered about human behavior on this planet. The phrases, in order of intensity, are the following:
You have to admit, there's a shade of variation among these expressions of disbelief, and if one is careful about nuances, as I like to tell myself I am, one must chose carefully among them to convey just the right degree of astonishment. In this particular case, I think option 5 is appropriate. (I have to admit that the occasions anymore of late for the use of 1. and 2. are limited.)
- I thought I had heard everything.
- Now I've heard everything.
- Nothing surprises me anymore. [in a resigned tone]
- Ya gotta be shitting me!