The Migrant Fleet is the largest concentration of starfaring vessels in the galaxy, sprawling across millions of kilometers. It can take days for the entire fleet to pass through a mass relay.
The Protheans left behind a series of relics and artifacts for the galaxy’s future beings to discover. When a race finally achieved space travel proficiency and discovered a marker left behind by the Protheans, they were directed to a mass relay—a piece of alien technology that allows faster-than-light travel.
2012 June, Stew Wilson, “Mass Effecting the Thousand Suns”, in RPG Review[2], number 16, →ISSN, →OCLC, page 43:
Humans, from the planet Earth, are the newest sentient species of notable size to enter the galactic stage and are hands-down the most rapidly expanding and developing. They independently discovered a Prothean data cache on Mars in 2148, and the mass relay networks shortly thereafter.
Alec Ryder, she knew from the dossier. Former Alliance marine—N7-ranked, no less. Enlisted just after the Prothean ruins were found on Mars, like so many young people of the time, eagerly wanting to be on the front lines of humankind’s next quantum leap. And he’d succeeded in that; Ryder had actually shipped out with Grissom, on that famous first flight through a mass relay!
