What caused this?
 

While pressure washing my block before assembly I noticed a chunk of metal missing next to cylinder 4 on the bottom.
My mechanic friend says someone must have dropped something heavy. I am thinking that area was weakened by detonation and with the water pressure it got knocked out.
Anyone else expereinced anything like this?

That is normal, that is how the cast is made.

I don't think that big hole is normal.

mazda caused that.

I said the same thing to myself the first time I was in a BP

That's where the oil drains back to the pan from the head. it is made that way.

That area is not exposed to detonation or water pressure. It's the oil drain for the cylinder head. 100% normal.

1) Totally normal
2) Find a new mechanic