בראשית ז:יא - בשנת שש מאות שנה לחיי נח בחדש השני בשבעה עשר יום לחודש ביום הזה נבקעו כל מעינת תהום רבה וארבת השמים נפתחו.
Regardless of the source of this water from below, its presence can answer two questions concerning the flood. One, 7:1-4 records G-d’s final instructions to Noah before the flood. This conversation was on the tenth of the month since G-d told Noah that the rain was to begin in seven days, and the rain began to fall on the 17th of the month, 7:11. 7:6 then records that the water from the flood were on the land, and 7:7 records that Noah and is family went into the ark due to the water of the flood. Both of these verses happened on the 10th of the month. Afterwards, 7:11 records that that all the fountains of the deep and of the sky opened on the 17th of the month. How was there water from the flood already on the earth in 7:6,7? Are 7:6,7 anticipating 7:11? A simple answer is that 7:6,7 are referring to the water rising from springs not so deep or that water from the depths of the earth was starting to percolate on the land before the water from below gushed forth and the rain began.
The appearance of this water in 7:6,7 does not contradict 7:4 since 7:4 only refers to the rain that would be in seven days, and this water was not rain but springs from under the ground. This water was not yet sufficient to flood the world, but still this water was part of the flood. Later, on the 17th, then 7:11 records that all of the springs from under the earth shot up since previously only a few of the springs had "sent" water to the surface.
A second question is that 7:24 records that the water on the land swelled for 150 days, but 7:4,12 (and 7:17) record that it only rained for forty days. How could the water swell if there was no more rain?
Ibn Ezra (on 7:24) explains that during the period of 150 days there was rain, but it was not continuous, as opposed to the initial 40-day period where the rain did not stop. He brings a proof from 8:2 which records that the rain stopped, and as this is recorded after the mention of the 150 days, this implies that there was rain after the initial forty days.
Ramban (on 8:4) seems to explain that during the 150 days the water was unable to evaporate but there was no new water. The water from the initial 40 days remained with their full destructive power for all the 150 days and this is what the phrase va-yigbaru ha-mayim in 7:24 means (see his comments on 7:18).
I doubt these answers. According to the Ramban, the water stayed at the same level, which means the water did not swell up. Ibn Ezra is correct that 8:2 implies that the rain continued after the forty days, but the question is whether the rain in this period was significant or not. If it was just regular rain, then this quantity could not be said to cause the water to swell. However, if it was a significant amount of rain, then why does the Torah state in 7:12 that it rained for forty days, when it according to Ibn Ezra it rained more than forty days even if the rain was not continuous?
My understanding is that when 8:2 records that the floodgates of the shamayim were stopped and the rain ended, this means that when the land was drying up, G-d stopped even normal rain from falling to not hinder the drying up process. With this understanding, 8:2 implies that after the forty days of very significant torrential rain, there was normal rain in the ensuing period, but this rain would not have been enough to cause the water to swell up. What then caused the water on earth to swell after the significant forty-day rain stopped?
One possibility is the extent of the flood was limited after the forty days of flood. The basic amount of water remained the same during the ensuing 110 days (or 103 days according to my understanding, see our discussion on 7:11- 8:19, The chronology of the flood), but then G-d started to limit the area covered by the flood, which would lead the water in the remaining areas to rise further. Admittedly, this would require another miracle of some "fence" to keep the water from running off, but the flood can only be understood by invoking miracles, and this miracle of a "fence" reduces the amount of water, which lessens the dependence on miracles to explain the amount of water. It was during this period, after the forty days, that the mountains were covered, but this was only in the smaller area that remained flooded. (Note this idea that the area under water contracted could be whether the initial flood of forty days was global or local.)
A second possibility is that while the torrential rains stopped after forty days, the water from the great depths of the earth continued rising up until the end of the hundred and fifty-day period. This water could not have been seen by Noah, even if he was awake in this period, since it was coming from below, but it would have caused an increase in the water level on earth. 8:2 records that after one hundred and fifty days the water from the depths of the earth was stopped up since until then this water had continued to rise and had caused the water level on earth to swell.
Bibliography:
Frances, Peter, 2017, Natural Wonders of the World, London: Dorling Kindersley (DK) Limited, a division of Penguin Random House LLC.