The box spins and emits green sparkles, it has no texture and it resides at the end of the Lava Cavern. It's current purpose is unknown, however, many theories have come of it, and Defaultio hasn't stated what its purpose is for.
The box, when immediately touched, states "Your success has been noted."
The music around the box is also the same as the River Tunnel's music. This may mean that the River Tunnel has a link to the green box in the future.
As of recent reports, the Green Box has been accessed many times, and very soon it may reveal its upcoming secret.It should be said that the box actually isn't Green but rather White, and the ambiance is making the box become green in color.
Previously, the area around the Green Box was safe, and players cannot take damage. Now, as of recent observations, you take damage in that area.
There are two known ways to access the box, both are extremely hard, and take a huge amount of effort and teamwork. Some have even given up on obtaining or getting inside this feature and have discarded the idea of it even being possible.
Lava Cavern Pathway: Edit
This pathway is widely known because of it being shown in the Volcano Obstacle, against the wall a tiny path leading down into an area. This leads directly to the box, however, pits of lava will stop you in your tracks. To get past these pits, you need long planks. Some use Elm Wood with a Sawmax, and put it on a low setting (the recommended is 1.6 x 0.4, but you can go higher. It's dependent).
Once you have a plank, drive into the area where the path is (you cannot bring your car/truck down there), and bring the plank with you. You must act quickly because your health is draining! Only 1 plank can be done at a time.
It may take 5 planks approximately to get across, depending if you dropped any.
Boulder Cave Pathway: EditThis pathway was just discovered recently by two anonymous users who pushed themselves through the Boulder Cave. This option isn't really recommended unless you tried the first option first because it's very complicated. In order to go this way, you must first enter into the Underground Grove. Once you've gotten inside, you must get to the wall that has the green box, which is usually the wall right next to the big pit of lava. Once you've gotten above that wall, through glitches or with another person, you can continue into the path shown in the picture to the right. Your health still drains here, but not as much, so still act quickly. Take the left-hand side of the wall, and jump across to the right (you can see it in the picture). Once you jump across, go around into the crawlspace and jump down the hole against the wall, and then you've made it to the box.
To see the video for the Underground Grove, look to the right.
[NOTE: This pathway was attempted to be fixed by Defaultio by placing a wall over the roof of the Green Box. However, the attempt backfired, as there was still a tiny crevice that wasn't patched, making the Green Box still accessible through this pathway. Therefore, this method is not patched.]
Mad Games Glitch Method Edit
This method is similar to the Underground Grove method. However, instead of going through the boulder cave, you wall glitch through the front face of the volcano. The great thing about this method is that it is possible to solo it.
Latest Confirmation of 7/14/16, see here.