Sons of God

What are the Nephilim? This is new to me. At the beginning of Genesis 6, there is this bizarre and all-too brief reference to the Nephilim, also referred to as the “sons of God,” who married the “daughters of men” and had children who grew up to become the “heroes of old, men of renown.”
It sounds like the authors of the Old Testament picked that bit up straight out of Greek mythology (aka Hercules). A quick Google search reveals much speculation about what this passage meant. Some think it refers to fallen angels who mated with human females and created a race of giants and superheroes like Samson and Goliath.
The passage also says that they “were on the earth in those days—and also afterward” giving the impression that they somehow survived the flood since they are referenced again later on in the Bible. In the middle of the passage, we also have God suddenly deciding to limit the maximum age of man to 120 years, so no more Methuzalahs after that. But why did God do that and was it supposed to have something to do with the Nephilim? One can only speculate.