Geometrician Jumpgate

A Geometrician Jumpgate, also known as an Underzone Portal, is a device for accessing the Underzone, a basement universe in which gateways to stars potentially lightyears distant are brought close together into a "transit hub" of sorts. Like the Underzone itself, it is thought these gates were created by the Geometricians.
A Jumpgate is similar in concept to a conventional stargate in that it contains a distortion in the spacetime metric which connects two distant points—across two separate universes, in this case. Beyond that, similarities break down. Jumpgates are clearly a product of technology almost unfathomably beyond that of the modern Starweb: each is a thin ring, several hundred to over a thousand kilometers across, massing more than a gas-giant moon, and rotating like a smoke-ring to create an aperture at the center.
Beyond this, precise physics and details are unknown. Ships seem to be accelerated to near-light speeds before passing into the Underzone, only to arrive there nearly at rest. The aperture itself resembles a series of nested spacetime "funnels", allowing entry through one end and exit from the other. Explorers attempting to take non-standard trajectories through a Jumpgate, passing not through the center of the "funnels", but a side instead, sometimes emerge from the "wrong" gate in the Underzone...or are just never seen again.