So many people showed up on Saturday, there were road blocks, traffic jams, and a wait of more than two hours for some people to get inside. Some didn't seem to mind, though.
"Maybe it's been a little over an hour, but it's been really pleasant, wonderful weather," said Craig Anderson, visitor.
"We went over there and enjoyed some of the other things because the line was so long, I just said we'll just come back over here," said Anthony Travis, San Francisco resident.
There were things to keep people entertained while they waited - one worker brought a snake around for children to see up close. The facility, itself, took three years to build, at a cost of $480-million. The hilly roof is covered with plants that are native to California - it spans two and a half acres.
"I've never seen anything like this before, it's just so amazing that they did this," said Ariel Sullivan, visitor.
Inside the building, you can find the Kimball Natural History Museum, the Steinhart Aquarium and Morrison Planetarium, as well as eight scientific research departments. One of the main attractions is a huge albino alligator named Claude who lives in an area called 'The Swamp.'
The Academy of Sciences will begin charging admission on Sunday. But you'll still be able to see the museum for free, every third Wednesday of the month - indefinitely.