Project Arcturus is capable of generating astronomical amounts of energy- Dr. Rodney McKay speculated that at only 50% maximum output, it had power equal to a dozen Zero Point Modules. Unlike a ZPM, which extracts Zero point energy from an artificially-created region of subspace, Arcturus was an attempt at extracting zero point energy from our own universe. In the words of Dr. Radek Zelenka, this could potentially make it as powerful as the scope of the universe itself. It was described as the Ancients' last great discovery after ascension and the Stargates. (SGA: "Trinity")
The core of Project Arcturus is a small white sphere with six spikes inside a large space. The region is surrounded by a powerful containment field, and Arcturus extracts Zero point energy from within this region.
The Atlantis team later revised the process to try and extract Zero point energy from an alternate reality via a Matter bridge, which would solve the exotic particle problem that plagued the original project. (SGA: "McKay and Mrs. Miller")
The Doranda outpost featured a powerful weapon capable of destroying a Wraith Hive ship with ease. Unlike many other weapons, as it is directly connected to the power source, it can continuously fire rounds and hence destroy ships easily. An Asgard shield can withstand at least a limited number of hits from the weapon. The weapon protected Doranda from Wraith attack and took out an entire fleet of ships. (SGA: "Trinity")
Circa 10,000 years ago
The scientists who were working on the project did not feel that it was ready to be tested under battle conditions, but they had no choice, and Arcturus failed. Extracting Zero point energy from a region of space randomly and drastically altered the laws of physics within that region. This resulted in the continuous creation and destruction of exotic particles within the containment field. As they were created in a region with radically different physical laws, their behaviour could not be predicted in this space/time and the containment field reacted with and could not properly contain them - when the containment field began to fail they would breach as hard radiation.
During the original test, the Dorandan weapons acted as a release valve in an attempt to prevent a catastrophic overload - using up the excess power by shooting randomly into space. The Ancients working on Arcturus barely managed to shut down the weapon, but it was too late; it had already obliterated the planet's surface and the Wraith fleet in orbit. Due to the automatic containment field adjustment system the Ancients had in place, the overload finally ended on its own with a massive release of hard radiation, which killed the researchers nearly instantly. (SGA: "Trinity")
The Atlantis expedition discovered the Stargate address of Doranda in the city's database. After the discovery of the Doranda outpost still intact on the planet, Dr. Rodney McKay and a team of scientists attempted to finish the Ancients' work, not knowing about the particle containment problem.
In their first power-up, an overload occurred, and the containment field shifted asymmetrically in the direction of the command access tube, killing Dr. Collins who was attempting to manually adjust the containment field. Despite protests from Dr. Elizabeth Weir and various other scientists, McKay was allowed to attempt one last power-up of Arcturus, which he attempted to do by trying to manually adjust the field to compensate for the particles. His attempt failed, and resulted in another overload, which was significantly more catastrophic than the two previous ones. The weapon armed and began firing at random targets to prevent a catastrophic overload.
Thanks to intervention from the Daedalus, McKay and Lt. Colonel John Sheppard managed to escape with their lives, but it was too late for Arcturus. Because McKay had disabled the containment field monitoring system, and was adjusting the field manually, there was nothing left in place to automatically shut it down after they escaped. The power source overloaded and caused a massive explosion, destroying Arcturus itself.
While being reprimanded by Weir for destroying "three-quarters" of a solar system, McKay (in his classic pompous precision) objected that it was actually "five-sixths, but it's not an exact science." However, most people simply refer to McKay's failure as "destroying a whole solar system". (SGA: "Trinity")