In Acts 16, what opened the prison doors while Paul and Silas were singing hymns at midnight?

Answer: An earthquake

About midnight, as Paul and Silas prayed and sang hymns, a strong earthquake shook the prison; all the doors flew open and chains came loose.

“About midnight Paul and Silas were praying and singing hymns to God, and the other prisoners were listening to them. Suddenly a strong earthquake shook the foundations of the prison. At once all the doors flew open and everyone’s chains came loose.”— Acts 16:25-26 (BSB)