  It was recently shown that the extremal charged black holes in STU
supergravity suffer from superradiant instability owing to the existence of the
unstable (low-frequency) quasibound states associated with a charged massive
scalar field. In this paper, we show numerically that for some charge
configurations, these black holes can also excite the (higher-frequency)
superradiantly unstable quasinormal modes. We find empirically that the
unstable modes are closely related to having a volcano-shaped effective
potential in the Schr\"odinger-like wave equation.
