Nephilim (fallen angels) after this time of Noah's flood. The giant named Goliath, which David slew, was one of them.
     Did you notice that The LORD Himself shut them in the Ark? As many of these flood events of Noah's time are used to compare with the latter day events, God closing Noah's ark and sealing it is a pointer to the sealing of His latter day servants mentioned in The Book of Revelation:

Rev 7:2   And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea,
3    Saying, "Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads."

     Noah's flood is a symbol for Satan's flood of the end time tribulation. Just as GOD closed Noah's Ark and sealed it from the waters of the flood entering in, likewise He has commanded that none of His today which He seals with His Truth are to be harmed.
     Well if 'two of all flesh' came aboard Noah's Ark along with Noah's family, meaning also two of every race of mankind, does that mean those other races not of Noah's family were 'sealed' by God also?

 

Rev.7: And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor on the sea, nor on any tree. 4 And I heard the number of them which were sealed:  an hundred and forty and four thousand of all the tribes of the children of Israel.14 ...these are they which came out of great tribulation, and washed their robes, and made them white in the blood of the Lamb. 17 For the Lamb Which is in the midst of the throne shall feed them, and shall lead them unto living fountains of waters: and God shall wipe away all tears from their eyes.

 

... After all the 144,000 representing the twelve tribes of Israel are sealed, we have this very next verse:

Rev 7:9   After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;

     These are the Gentile believers of Christ's future Kingdom, from all nations, kindred’s, and people, and tongues. And the number of them were so many, that no man could enumerate (arithmeo, to count) them.
     Back to Gen.7:

Gen 7:17   And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift up above the earth.
18    And the waters prevailed, and were increased greatly upon the earth; and the ark went upon the face of the waters.
19    And the waters prevailed exceedingly upon the earth; and all the high hills, that were under the whole heaven, were covered.
20    Fifteen cubits upward did the waters prevail; and the mountains were covered.

     These 'high hills' and the 'mountains' which were covered with waters has another Message also. The 'high hills' are symbolic of the refuge of the 'wild goats', in nature, and for how Satan's own take refuge in the world with false worship

 

Ps.104:18: The high hills are a refuge for the wild goats; and the rocks for the conies.

 

 Jer.17:2: Whilst their children remember their alters and their groves by the green trees upon the high hills.