  Highly charged ions (HCIs) are insensitive to external perturbations and are
attractive for the development of ultrastable clocks. However, only a few HCI
candidates are known to provide optical clock transitions. In this Letter, we
show that, as a result of strong relativistic effects, there are more than 100
suitable optical HCI clock candidates in more than 70 elements. Their
transitions are embedded in the fine-structure splitting of the $nd^4$, $nd^5$
and $nd^6$ ground-state configurations with $n=3,4,5$ being the principal
quantum numbers. The corresponding high multipolarity transitions in these ions
have lifetimes and quality factors many orders of magnitude longer and larger,
respectively, than those in state-of-the-art clocks. Their polarizabilities are
also orders of magnitude smaller, rendering them more stable against external
electromagnetic fields. Furthermore, within the same electronic configurations,
the clock transitions in heavy ions scale up to the XUV and soft-x-ray region,
thus enable the development of clocks based on shorter wavelengths. The
existence of multiple clock transitions in different charge states of a single
element, as well as in a whole isoelectronic sequence, would significantly
enrich the detection of fine-structure constant variations, the search for new
physics and the test of nuclear theories via high-precision spectroscopy.
