We convert a strongly interacting ultracold Bose gas into a mixture of atoms and molecules by sweeping the interactions from resonant to weak. By analyzing the decay dynamics of the molecular gas, we show that in addition to Feshbach dimers it contains Efimov trimers. Typically around 8% of the total atomic population is bound into trimers, identified by their density-independent lifetime of about 100 µs. The lifetime of the Feshbach dimers shows a density dependence due to inelastic atom-dimer collisions, in agreement with theoretical calculations. We also vary the density of the gas across a factor of 250 and investigate the corresponding atom loss rate at the interaction resonance.Experiments with ultracold atomic gases provide access to a vast array of intriguing phenomena, in part because of magnetically tunable Feshbach resonances. In particular, recent experimental [1][2][3][4][5] and theoretical [6][7][8][9][10][11][12][13][14][15][16] advances have made resonantly interacting Bose gases an exciting new research topic [17]. Unlike their fermionic counterparts, strongly interacting Bose systems are profoundly influenced by three-body phenomena, and help us understand the progression from two-through few-to many-body physics.At the Feshbach resonance the s-wave scattering length a diverges, and in the case of zero density the Feshbach molecule state, also of size a, merges with the free-atom state. This diatomic resonant scenario is the prelude for a set of exotic few-body phenomena, namely the Efimov effect. Although the Feshbach molecular state is unbound at the resonance, there exists an infinite log-periodic series of Efimov three-body bound states [18,19]. At 1/a → 0 the size of the p th Efimov state (p = 0, 1, 2...) is larger than the previous by a factor by 22.7, and its binding energy ET smaller by a factor of 22.7 2 [20,21]. At finite density n many-body effects complicate the physics. The system has an additional length scale, the interparticle spacing n −1/3 , that may determine how few-and many-body interactions scale. Many questions arise, such as: what are the structure, strength, length scale and dynamics of the two-, few-and many-body correlations? What does it mean to have a two-or three-atom molecule when it is embedded in a gas with interparticle spacing comparable to the molecular size?Both the ambiguous constitution of two-and three-body states in a many-body environment and the short-lived quasiequilibrium of a resonantly interacting Bose gas [3] complicate experiments. For these reasons, many experiments simplify matters by reducing interactions to a well-understood regime before imaging [1][2][3][4]. This interaction sweep can preserve resonance fossils in the form of perceived loss [1, 2, 4], momentum generation [3], and molecule formation.In this letter, we create a mixture of 85 Rb (free atoms), 85 Rb * 2 (Feshbach dimers), and 85 Rb * 3 (Efimov trimers) by sweeping a resonantly interacting degenerate Bose gas onto the molecular states in the weakly-interacting regime (na 3 1). O...