When thou didst terrible things - In delivering the people from Egypt, and in conducting them to the promised land.
Which we looked not for - Which we had never before witnessed, and which we had no right to expect.
Thou camest down - As on Mount Sinai.
The mountains flowed down - (See the notes above). The reference is to the manifestations of smoke and fire when Yahweh descended on Mount Sinai (see Exodus 19:18).