We checked out Amazon's new skyscraper HQ - including the giant glass spheres

In November 2016, Amazon opened "Day 1" - a 521-foot-tall skyscraper in the heart of downtown Seattle that will serve as its new base of operations.

The most well-known feature of the Day 1 tower is actually outside: A set of three gigantic, glass spheres that will be filled with greenery, including endangered species, when they are completed in 2018.

The idea is to give Amazon employees something of an oasis in the middle of the company's famously demanding corporate culture.

The last time Business Insider swung by the Day 1 location in August 2016, the tower was receiving its finishing touches and the spheres were only half-completed.

I made a return visit in early May while in Seattle for the Microsoft Build conference. There's been a good deal of progress and the project looks impressive. Here's a look at the Day 1 tower and the mysterious glass biospheres that have sprouted up at its base: