John Gill Commentary Acts 16:26

"and suddenly there was a great earthquake, so that the foundations of the prison-house were shaken: and immediately all the doors were opened, and every one`s bands were loosed." — Acts 16:26 (ASV)

And suddenly there was a great earthquake; An extraordinary and unusual one; which did not arise from natural causes, from wind being pent up in the earth, but from the prayers and praises of the saints, going up to heaven; when God was pleased to testify his presence and power this way; and the effects which followed upon this earthquake, show it to be a supernatural one.

so that the foundations of the prison were shaken; not the roof and walls only, as is common in earthquakes, but the very foundation also; and yet the edifice was not thrown down, as is usual when the foundation is shaken.

and immediately all the doors were opened; both of the outer and inner prisons, which is another unusual effect of earthquakes.

and everyone's bands were loosed; not only the bands of Paul and Silas, but of the rest of the prisoners; though the Arabic version reads, "all the fetters and bands of both were loosed", referring it only to Paul and Silas; this circumstance shows also, that the earthquake was miraculous, for when was it ever known that such an effect ever followed one?