USS Equinox's shields stressed by interspatial fissures
An interspatial fissure is an opening between dimensions, used by nucleogenic lifeforms to enter and exit this space-time continuum.
Nucleogenic lifeforms can open interspatial fissures anywhere they wish, except for areas protected by force fields. However, the fissures will weaken the shields if opened in close proximity. In 2375, nucleogenic lifeforms attacked the USS Equinox and the USS Voyager in this manner, opening hundreds of fissures to wear down the starships' defenses. (VOY: "Equinox", "Equinox, Part II")