Boomtubes are interdimensional wormholes that allow people to traverse the various demesnes of The Nine Realms and other dimensions. Boomtubes are the only mechanism that allow travel between these worlds, and serve to project users to any planet in the Demiurge's 'network', called Yggdrasil by Asgard.
Boomtubes are far from foolproof, however. They emit extremely distinctive neutrinos that can be detected from significant distance. Anyone who travels through a boomtube can be easily detected for quite some time because of these unusual neutrino emissions, and there is no effective means of stopping this radiation.
Boomtubes are not wholly reliable. Rapid, repeat use of a boomtube or substantial transfer of matter can cause boomtubes to destabilize. This can result in destination or origin points disconnecting from one another, with the dislocation of anyone in transit. Boomtubes can also be obstructed by a charged particle field, preventing the destination from forming and ejecting the passenger mid-transit. This prevents boomtubes for purposes of espionage or military activity. Lastly, they cannot stabilize an exit node reliably outside of the influence of the Demiurge's reach, making them unsuitable for deep space exploration.
See: Space Travel