We checked out Amazon's new skyscraper HQ — including the giant glass spheres (AMZN)

The most well-known feature of the Day 1 tower is actually outside: A set of three gigantic, glass spheres 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:

This is Day 1 — Amazon's new skyscraper headquarters, which opened in November 2016. It's 37 stories tall. The name refers to Amazon CEO Jeff Bezos mantra that "it's always day 1;" a reminder to never rest on your laurels.

As you approach the Day 1 tower, you may notice a message on its side. From up close, it's kind of hard to make out...

...but if you step away, you can see that it says "Hello World," a reference to the classic example used to demonstrate new programming languages or concepts.

The building also houses Amazon Go, the retailer's version of a convenience store of the future. Right now, it's only open to Amazon employees, with no timeline for opening to the public.

More on Amazon Go

What makes the store so special is that there are no cash registers or lines — it uses cutting-edge computer vision and payments technology so that you can just walk out with whatever you want.

But far and away, the most impressive part of the Day 1 campus are the famous Spheres: Three gigantic glass domes that will be filled full of exotic and endangered flora.

Not to put too fine a point on it, but they're huge. They range from 80 to 90 feet tall.

The domes are impressively massive. Here are some workers on the outside of the dome, for scale.

Last time we visited, in the summer of 2016, this one on the end hadn't had all of its glass installed. Now, things are looking good.

It's difficult to see with the mirrored sides, but if you squint, you can see that Amazon has started to fill the sphere with its first few plants.

(Here's an official Amazon photo of the first plant going into the soil from the inside.)

When they open in 2018, the domes are intended to be the center of Amazon employee life. Outside the domes, you can already see some of the green space and weather-shielding canopies already in place. The domes are only for employees, but these areas are for anybody.

And the sphere-area dog park is already in full operation.

And in case you were wondering about the internals of the Day 1 tower, here's a look inside, courtesy of Amazon. This is the lobby area, when you first walk in:

There's an airy common area for Amazonians to share a cup of coffee, featuring the backside of the "Hello World" sign.

Notice how the lighting is all done up in Amazon black-and-orange.

Amazon tries to put some green space inside the building, too.

The hallways are austere and modern.

Finally, in case you were wondering, the name "Day 1" itself refers to Amazon CEO Jeff Bezos' famous philosophy about never resting on your laurels: "Staying in Day 1 requires you to experiment patiently, accept failures, plant seeds, protect saplings, and double down when you see customer delight."