To be fair, the church was there first: The Russian Orthodox Cathedral of Christ the Savior was commissioned by Alexander I in 1812 and completed in the 1860s, when Tchaikovsky's "1812 Overture" held its magnificent premiere there. But a magnificent church was a tempting target for Soviet ministers looking for a new symbol to unite Russia, and Stalin ordered it destroyed in 1931. A planned monument to the joy of socialism was never constructed, but instead Nikita Khrushchev put up something everyone in the city could usea splendid swimming pool.
When the Soviet Union officially called it quits, the Russian Orthodox Church got approval to destroy the pool and, despite the underground streams that fed it, rebuild the cathedral in the height of its former glory. The decor inside is splendid, but the plaza the church sits on is still distinctly pool-y; you'll notice odd dips and what look like entrances going underground. It's as if the builders couldn't take the time to wipe out all traces of their secular, splashing childhoods.
If possible when you visit, borrow a Russian (and a translator) who remembers the pool days to take you around the site: Ours mesmerized us with tales of intricate tiles and heated tunnels leading from dressing room to pool so Muscovites could swim well into the fall.