The transient X-ray source HLX-1 has been hailed as the first confirmed intermediate-mass black hole (M ~ 10,000 solar masses, peak luminosity ~ 1E42 erg/s). If confirmed, it would provide precious observational constraints to models of black hole growth and galaxy assembly. However, many observed properties of HLX-1 still do not make sense or seem to contradict each other: we still do not know what this accreting system really is. We do not know the size of the system, the binary period, the source of gas, the reason for its recurrent outbursts, and its relation with an apparent host galaxy nearby. We need new ideas to find an explanation for this source.