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Acting as a tractor beam, the Excursion Funnel is powered by a round emitter placed on a wall, containing three gleaming, blue orbs, that emit strange blue/orange sparkles that fade as the funnel goes on. From each orb leaves a thread that swirls on itself in a spiral, the three threads making up the Funnel itself, a seemingly infinite transparent blue cylinder. Combined with portals, it can transport Test Subjects or objects to otherwise unreachable areas.
"Asbestos is harmless!" appears to be the Excursion Funnel's slogan, and a registered trademark of Aperture Science.
In at least one level of the Portal 2 single-player campaign, if the player is about to die whilst having portals redirecting an Excursion Funnel, the game corrects for placing the incorrect portal to save themselves.
The mechanics of the "liquid asbestos" aspect of excursion funnels is currently unknown. It is likely that the device generates a type of tractor beam, which propels a liquid mixture of several elements, one of these being liquefied asbestos.
Behind the scenes
The Excursion Funnel was first revealed through a diagram in the April, 2010 issue of Game Informer. In the diagram, a 1500 Megawatt Aperture Science Heavy Duty Super-Colliding Super Button serves as a placeholder for the Funnel's emitter. For an updated (and moving) diagram revealed during E3 2010, the Button was replaced by the definitive device.
By walking into an Excursion Funnel aimed toward the ceiling, waiting until fully stopped at the top, crouching and waiting again, then pressing jump while holding crouch, the player can float across the ceiling and possibly beyond if nudged correctly.
If an Aperture Science Sentry Turret is placed in contact with the Excursion Funnel while being held by the player, it will immediately freeze in its last position. When it is removed from the Excursion Funnel, the turret will enter sleep mode as if it isn't being held, and is even capable of shooting the player through portals
So far it is unknown why, while the Excursion Funnel was already in use in 1998, it is not seen in the first Portal, set over 20 years later.
An updated diagram.
A more updated diagram, showing a horizontal funnel.
↑Dave Saunders states in the Developer commentaryExcursion Funnel "As a solution, we now detect when the player places the wrong portal in hopes of saving themselves; we help them out by moving their other portal under the excursion funnel source."