Psalms 18:15 - Joseph Benson’s Commentary on the Old and New Testaments

Then the channels of waters were seen This is a description of the effects of the earthquake, by which the earth was rent in sunder, and such clefts made in it that the subterraneous passages of the waters were discovered, as has frequently been the case in violent earthquakes, whole rivers of waters sometimes issuing from the clefts, and spouting up a great height into the air. The foundations of the world were discovered That is, Such large and deep chasms, or apertures, were made by the violence of the shock that the lower parts of the earth were laid open to view, and made perfectly visible.