Half of the condensate has been cut away to expose the centre. a, Miscible superfluid of 50,000 rubidium and 50,000 sodium atoms, with a rubidium-sodium scattering length A12 = 1.8 nm. b, Immiscible superfluid, with rubidium atoms inside a shell of sodium atoms; A12 = 9.6 nm. c, As b, but the higher-energy metastable state is shown, with the sodium on the inside. d, As b, but now 200,000 sodium atoms compress 50,000 of rubidium into a far smaller volume. These pictures are from our simulations, with parameters the same as those used by Pu and Bigelow1.