The Mars Archives describe a superweapon that the Alliance has named the Crucible, which exploits the technology of mass effect relays. Beyond the basic principles, however, researchers know little about how the weapon actually works.
One popular theory suggests that since relays can transfer matter and energy across the galaxy with little regard for distance, it may be possible to create a weapon for which range is barely a factor. Duplicating the advanced science used to build the relays has proven difficult, however. If the Crucible were completed, the challenge would become tuning the weapon to kill a Reaper halfway across the galaxy without inflicting unthinkable levels of collateral damage.
What is clear is that the Crucible's construction is a massive effort, drawing resources from throughout explored space. Staggering financial costs have been disregarded in the common effort to create something, anything, that can stop the Reapers.