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Axions are some of the best motivated particles beyond the Standard Model. Weshow how the attractive self-interactions of dark matter (DM) axions over abroad range of masses, from $10^{-22}$ eV to $10^7$ GeV, can lead tonongravitational growth of density fluctuations and the formation of boundobjects. This structure formation enhancement is driven by parametric resonancewhen the initial field misalignment is large, and it affects axion densityperturbations on length scales of order the Hubble horizon when the axion fieldstarts oscillating, deep inside the radiation-dominated era. This effect canturn an otherwise nearly scale-invariant spectrum of adiabatic perturbationsinto one that has a spike at the aforementioned scales, producing objectsranging from dense DM halos to scalar-field configurations such as solitons andoscillons. We call this class of cosmological scenarios for axion DM production"the large-misalignment mechanism."  We explore observational consequences of this mechanism for axions withmasses up to $10$ eV. For axions heavier than $10^{-5}$ eV, the compact axionhalos are numerous enough to significantly impact Earth-bound direct detectionexperiments, yielding intermittent but coherent signals with repetition ratesexceeding one per decade and crossing times less than a day. These episodicincreases in the axion density and kinematic coherence suggest new approachesfor axion DM searches, including for the QCD axion. Dense structures made up ofaxions from $10^{-22}$ eV to $10^{-5}$ eV are detectable through gravitationallensing searches, and their gravitational interactions can also perturbbaryonic structures and alter star formation. At very high misalignmentamplitudes, the axion field can undergo self-interaction-induced implosionslong before matter-radiation equality, producing potentially-detectablelow-frequency stochastic gravitational waves.

The Large-Misalignment Mechanism for the Formation of Compact Axion Structures: Signatures from the QCD Axion to Fuzzy Dark Matter