At one point, the pre-flood world had 120 more years to repent. 120 years to turn to God. 120 more years of mercy (Genesis 6:3). Those years were now gone. The clock was ticking and the countdown had almost hit zero.
Now there were no more years. Now there were not even anymore days. There were but mere seconds. The ark was loaded. Everything was in. There was just one more thing left to do— close the ark. Noah, however, did not have to do this final task:
Those that entered, male and female, of all flesh, entered as God had commanded him; and the LORD closed it behind him. (Genesis 7:16)God Himself closed the ark. What exactly God closed is not specified. He likely closed the door of the ark, and perhaps the window of the ark (which Noah opens in Genesis 8:6). God also could have closed the covering of the ark (which Noah removes in Genesis 8:13).
Whatever part of the ark was closed, God closed it with a resounding roar. The sound of the roar echoed violently across the entire pre-flood world. The aftershocks of it affected every man, woman, and child. God had shut Noah in the ark... which meant that the pre-flood world was officially shut out of the ark. Time had expired. There were zero more years of mercy— and zero more seconds of mercy.
Zero More Years of Mercy (Part 2) - Genesis 7:16
The Destruction of The City of Enoch (Part 1) - Genesis 7:17-18
Noah's Timely Entry (Part 1) - Genesis 7:13-15
State of the World (2349 BC): The Flood (Part 1) - Genesis 7:11
120 More Years of Mercy - Genesis 6:3