Complex sentence: Bryant Park is located entirely over an underground structure that houses the library's archives, which were built in the 1980s when the park was closed to the public and excavated; the new library facilities were built below ground level while the park was restored above it.
Split into simpler sentences: Bryant Park is built entirely over an underground structure that houses the library's archives. In the 1980s, the park was closed to the public and excavated, and the new library facilities were built below ground level while the park was restored above it.