Most interiors get some help from the outside. A window sash that doesn't quite seal, a door to a corridor, a mechanical system pulling in fresh air on a schedule. Whatever a material gives off, the building dilutes.

An underground library gets none of that for free.

Phrontisterion — Mona's new library, which opened in June 2026 — sits beneath the inverted ziggurat of Anselm Kiefer's concrete amphitheatre, linked to the museum by sandstone tunnels. Four years of construction. More than $100 million. Around 30,000 books from David Walsh's personal collection. Designed by Nonda Katsalidis.