The Enterprise's sensors registered the cube as entirely solid, each edge 107 meters long, and massing just under 11,000 metric tons. But neither the sensors nor Montgomery Scott's experience were able to explain how the device sensed the ship, moved to block it, and moved to remain in position.
The cube held a position 1,593 meters in front of the Enterprise. Kirk elected to play a waiting game. Eighteen hours later, with no change in the cube's status, Kirk convened a briefing. Spock speculated that the device was either a space buoy, or "flypaper" – designed to hold ships in place, or to measure their capabilities. He conjectured it would be unwise to appear too weak.
The ship attempted to pull away, using a spiral course. The cube maintained its position but, at half-speed, began to emit dangerous radiation, and started to close on the Enterprise. Kirk first ordered a halt; when the cube continued to close and emit radiation, he retreated before it. It maintained its approach, closing even at warp factor 3 speeds, and emitting increasingly deadly radiation. It was finally destroyed, at point-blank range, by a short burst of phaser fire.
Events that followed revealed that the cube had been intended first to hold the ship, and later to threaten it, so that the alien Balok could learn what sort of reaction this would prompt. At one point, Balok used Kirk's destruction of the cube as "proof" that the Federation was a hostile organization; this was a ploy, on his part, to goad Kirk for the purpose of testing him. (TOS: "The Corbomite Maneuver")
The original version of Balok's cube was filmed at the Howard Anderson Company. (Cinefantastique, Vol. 27, No. 11/12, p. 76) As per Desilu's purchase orders to him (currently archived at the UCLA), the studio model was built by Wah Chang , whereas the model was made to appear functional by the studio's own special effects staffers Jim Rugg and Roger Dorney.  Behind-the-scenes imagery has revealed that the design was, onstage, simply referred to as "cube" and that the model was being filmed at Anderson's at the end of June 1966.
When the buoy was digitally rendered for the 2006 remastered version of the episode, it was intended that the cube not be drastically changed from its original appearance. VFX Line Producer Michael Okuda recalled, "We wanted CBS Digital to recreate that almost exactly." Fellow VFX Line Producer Dave Rossi noted, "We gave it a little more sense of dimensionality, I think." (Spacelift: Transporting Trek into the 21st Century, TOS Season 1 Blu-ray special features)
According to Star Trek Maps (chart B), these First Federation buoys were referred to as outpost buoys. There was a network of them with each buoy approximately ten parsecs apart from one another, at the border of First Federation space.
The design of Balok's cube – or conceivably even the (modified) model itself, considering the tight budgets the production had to adhere to – was reused as the Kalandan outpost computer in the third season episode "That Which Survives".
This type of buoy was seen again, en masse, in the video game Star Trek: Shattered Universe, this time belonging to the mirror universe Balok, used against the ISS Excelsior. Each individual buoy was dispatched by a few shots from the Excelsior fighters.