During Avengers (2012), at the beginning of the movie before Loki activates the Tesseract, Nick Fury and Hawkeye have the following exchange:
Hawkeye: No one's come or gone. And Selvig's clean. No contacts, no I.M.'s. If there was any tampering, sir, it wasn't at this end.
Nick Fury: At this end?
H: Yeah, the cube is a doorway to the other end of space, right? The doors open from both sides.
The simple question is, how did Hawkeye know what the Tesseract actually did, especially given that Fury and SHIELD were trying to use it primarily as a power source for civilian energy needs and military "Phase 2" WMDs? For that matter, did anyone else in SHIELD know for a fact that the Tesseract could generate and sustain wormholes?