FlyerTalk and Million Mile Secrets have found a 60k Mileage Plus explorer offer offering 50k miles on first purchase and 10k miles after $25k in spending. dabearz73 on FlyerTalk suggests logging into one’s United account and then going to the offer website.
I currently have the United Mileage Plus Select Visa Signature card and formerly had the Continental OnePass Plus Mastercard. I’m able to see the 60k offer when I’m logged in.