Pretty much the last thing you'd want to see out your window. And, conventiently enough, probably the last thing you would see at all.
The Planet Buster is a cannon the size of a moon, used by the Vok aliens in an attempt to destroy the prehistoric Earth.
After the Flying Island was destroyed in a battle against the Predacon forces, a beacon was sent to the Vok. Receiving this beacon, the Vok returned to the Earth and were quite displeased to see that the Beast Wars had completely wrecked their "project". Deeming the entire planet to now be a liability, the Vok revealed their magnum opus: a "termination sequence" that transformed the planet's hollow second moon into a massive weapon.
The Vok attempted to use this cannon (which Rattrap later dubbed "The Planet Buster") to ignite every energon deposit on the planet, resulting in an explosion powerful enough to destroy the planet itself!
As the alien device charged up, receiving power from an alien site on the planet, the power beam somehow caused the orbiting stasis pods to fall out of orbit.
In order to destroy the weapon, Optimus confiscated a customized stasis pod, which Tarantulas and Blackarachnia had rebuilt as a means to escape from the planet, and rigged its transwarp cell with an explosive charge.
Optimus' plan was to fly the Pod into the core of the Planet Buster, where the transwarp cell would explode, taking the alien construct with it. Unfortunately, the Pod itself was a trap, set by Megatron, revealed to Optimus when he was unable to exit the pod and escape from the imminent explosion. The plan to destroy the Planet Buster succeeded, but cost the Maximals their leader.
As an unforeseen development, the destruction of the Planet Buster created a massive quantum surge. This surge mutated several Transformers into Transmetals and damaged the stasis pods of others, resulting in the Fuzors and Transmutate. The destruction of the stasis pod's transwarp cell also created a transwarp wave which traveled across space and time, eventually reaching a Predacon outpost in the "present" time of the Beast Era, the time from which the Beast Warriors hailed. Upon intercepting this wave, the Tripredacus Council sent an agent to investigate.