  Recent advances in the fabrication and experimentation of Reconfigurable
Intelligent Surfaces (RISs) have motivated the concept of the smart radio
environment, according to which the propagation of information-bearing
waveforms in the wireless medium is amenable to programmability. Although the
vast majority of recent experimental research on RIS-empowered wireless
communications gravitates around narrowband beamforming in quasi-free space,
RISs are foreseen to revolutionize wideband wireless connectivity in dense
urban as well as indoor scenarios, which are usually characterized as strongly
reverberant environments exhibiting severe multipath conditions. In this
article, capitalizing on recent physics-driven experimental explorations of
RIS-empowered wave propagation control in complex scattering cavities, we
identify the potential of the spatiotemporal control offered by RISs to boost
wireless communications in rich scattering channels via two case studies.
First, an RIS is deployed to shape the multipath channel impulse response,
which is shown to enable higher achievable communication rates. Second, the
RIS-tunable propagation environment is leveraged as an analog multiplexer to
localize non-cooperative objects using wave fingerprints, even when they are
outside the line of sight. Future research challenges and opportunities in the
algorithmic design and experimentation of smart rich scattering wireless
environments enabled by RISs for sixth Generation (6G) wireless communications
are discussed.
