  Preparing and detecting spatially extended quantum superpositions of a
massive object comprises an important fundamental test of quantum theory. These
quantum states are extremely fragile and tend to quickly decay into incoherent
mixtures due to the environmental decoherence. Experimental setups considered
up to date address this threat in a conceptually straightforward way -- by
eliminating the environment, i.e. by isolating an object in a sufficiently high
vacuum. We show that another option exists: decoherence is suppressed in the
presence of a strongly interacting environment if this environment is
superfluid. Indeed, as long as an object immersed in a pure superfluid at zero
temperature moves with a velocity below the critical one, it does not create,
absorb or scatter any excitations of the superfluid. Hence, in this idealized
situations the decoherence is absent. In reality the decoherence will be
present due to thermal excitations of the superfluid and impurities
contaminating the superfluid. We examine various decoherence channels in the
superfluid $^4$He. It is shown that the total decoherence time can be as large
as tens of seconds for a $10^6$ amu nanoparticle delocalized over $300$ nm in
helium at $1$ mK. Performing interference experiments in superfluid helium can
provide certain practical advantages compared to conventional schemes, e.g.
compensation of gravity by the buoyancy force and effective cooling.
