Tafsir of Al-Mursalat 77:27

"And We placed therein firm mountains," meaning fixed, towering, and elevated mountains—from which the expression "he turned up his nose in pride" (shamakha bi-anfihi) is derived.

The description of a masculine plural by a feminine plural for non-rational beings is consistent, as in "known months" (ash-hur ma'lumat). Their use in the indefinite form is for magnification, or to indicate that in the earth there are mountains that have not been known or encountered; for the earth of Allah, the Exalted, is vast, and within it is what none knows except Allah, the Mighty and Majestic. It has also been said that it is to indicate that within the mountains is that which is unknown, namely the celestial mountains, which aligns with the proponents of modern philosophy, as they have asserted the existence of numerous mountains on the moon and conjectured their existence elsewhere. This has been criticized as an interpretation based on that which is not known.

"And We gave you to drink sweet water," meaning fresh water. This is because We created it at their (the mountains') bases, caused it to flow for you from them in rivers, and caused it to spring forth in sources that derive from what We have deposited within them. It may also be interpreted in a way more general than that, including the water sent down from the heaven.