Stanford University researchers fitted tags to the squid and tracked them as they dove through oxygen-poor waters off California during the day, returning to surface waters at night.
"We've seen them make really impressive dives up to a kilometer and a half (0.9 miles) deep, swimming straight through a zone where there's really low oxygen," researcher Julia Steward told the BBC.
"They're able to spend several hours at this kilometer-and-a-half-deep, and then they go back up and continue their normal daily swimming behavior. It's just a really impressive, really fast, deep dive through what is quite a harsh environment."
Humboldt squid, though predominantly found in waters off Mexico and further south, have recently been extending their range into California waters, where a band of low-oxygen water exists at a depth of more than 1,600 feet, the researchers said.
"It's amazing," Stewart said. "This is an animal you'd think would require a lot of oxygen and we see it swimming at pretty comparable rates to what we see it swimming in the highly oxygenated water.
"It seems they're somehow able to suppress their metabolism when they're in low oxygen, but they're by no means lethargic. They're swimming around quite well."