Complex sentence: In 1898, Paul Gout discovered, during some excavation under the floor of the church, Notre-Dame-Sous-Terre that was completely excavated in 1959, once the architect Yves-Marie Froidevaux had a concrete beam installed.
Split into simpler sentences: In 1898, during some excavation under the floor of the church, Paul Gout discovered the Notre-Dame-Sous-Terre. It was completely excavated in 1959, after architect Yves-Marie Froidevaux had a concrete beam installed.