We discuss the observational consistency, possible properties, and detection of collapsed nuclei $C_A$. These may be considered as elementary particles with mass number $A>\mathrm{}1\mathrm{}$ and of much smaller radius than ordinary nuclei $N_A$. The existence of $C_A$ of (perhaps much) lower energy than $N_A$ is observationally consistent if $N_A$ are very long-lived isomers against collapse because of a "saturation" barrier between $C_A$ and $N_A$. Barrier-penetrability estimates show that sufficiently long lifetimes $\gtrsim {10}^{31}$ sec are plausible for $A\gtrsim 16-40$. The properties of $C_A$ are discussed using composite baryon and quark models; small charges and hypercharges and, especially, neutral $C_A$ are possible. $C_A$ can be effectively a source or sink of baryons. Some astrophysical implications are briefly discussed, in particular the possible large scale presence of $C_A$ and the possibility that accelerated collapse in massive objects may be a source of energy comparable to the rest mass.

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