Today I'm going to write about my currently situation.

I'm a bit confused about the usage of You don't say. Sometimes it seems to be an expression of agreeing; other times it may just show surprise. Could any native English speaker do me a favor by listing all the possible meanings of this phrase and give examples? Thank you!

On April 20th 2010, an oil rig in The Gulf of Mexico exploded, spewing thousands of barrels of oil into the coastal water a day. The pip was finally cap 87 days later.

This oil spill contaminated 75 billion cubic yard of water over 113 days, destroying billion of fishes, oyters, invertebrate species, and thousand of birds and dolphins. It also have damaged many habitats.