A wormhole only centimetres across, too small for a ship, through which Voyager contacted a Romulan scientist in the Alpha Quadrant and then learned that it also led twenty years into the past.
The micro-wormhole is an ancient, decaying wormhole found by Voyager in the episode Eye of the Needle in 2371. Its opening is far too narrow for a starship, but a probe lodged inside can relay signals, and the far end proves to lie in Romulan space in the Alpha Quadrant.
The crew make contact with Telek R'Mor, a Romulan scientist aboard a lone survey vessel, who at first suspects a Starfleet trick. As trust grows, Torres finds a way to send a transporter beam through the passage, raising the hope that the whole crew might simply beam home.
When R'Mor is brought aboard as a test, Tuvok discovers the flaw: the wormhole is also a rift in time, and R'Mor comes from 2351. Going back with him would rewrite two decades of history. He returns alone, carrying the crew's letters to deliver at the proper time, but Tuvok later finds that he died in 2367, four years too soon.