The planetarium is one of the buildings that Dr. Bodkin remembers from his youth living in London, and Strangeman believes that Bodkin has treasure of some sort hidden there. While the lagoon is full, the planetarium is a symbol of potential and rebirth. It holds the potential for treasure, and for Kerans during his dive, it holds the potential for him to die, embrace what he sees in his dreams, and descend into the "time seas." However, after Strangeman drains the lagoon, the planetarium takes on the exact opposite symbolism. The sight of it filled with mud is one of the major reasons why Kerans is horrified by the new world before him, as it's no longer an incubator for life, but proof that Strangeman's hope for the world is in direct opposition to Kerans's vision of a future in which humans have ceded their spot as the dominant species.
The Planetarium Quotes in The Drowned World
"Dr. Bodkin, did you live in London as a child? You must have many sentimental memories to recapture, of the great palaces and museums." He added: "Or are the only memories you have pre-uterine ones?"
For some reason the womb-like image of the chamber was reinforced rather than diminished by the circular rows of seats, and Kerans heard the thudding in his ears uncertain whether he was listening to the dim subliminal requiem of his dreams.
No longer the velvet mantle he remembered from his descent, it was no a fragmenting cloak of rotting organic forms, like the vestments of the grave. The once translucent threshold of the womb had vanished, its place taken by the gateway to a sewer.