  We initiate a study on various cosmological imprints of string axions whose
scalar potentials have plateau regions. In such cases, we show that a delayed
onset of oscillation generically leads to a parametric resonance instability.
In particular, for ultralight axions, the parametric resonance can enhance the
power spectrum slightly below the Jeans scale, alleviating the tension with the
Lyman $\alpha$ forest observations. We also argue that a long-lasting resonance
can lead to an emission of gravitational waves at the frequency bands which are
detectable by gravitational wave interferometers and pulsar timing arrays.
