A nucleogenic lifeform emerges from an interspatial fissure
An interspatial fissure is a physical passageway through which the nucleogenic lifeforms encountered by the crew of the USS Voyager in 2376 gained access to the normal spacetime continuum.
The lifeforms could not open up the fissures in an area protected by shields, but could focus their efforts on weakening shields by opening a fissure within one meter of the shield perimeter. The lifeforms used this tactic when attacking Voyager and the USS Equinox, causing the shields to weaken by 0.3 percent every time a fissure opened. Working together, the two Starfleet crews devised a latticework of multiphasic force fields throughout both ships, which had the capability of denying the lifeforms the ability to open a fissure with which to return to their own realm. The aliens eventually ceased their attacks when the Equinox was destroyed. (VOY: "Equinox", "Equinox, Part II")