Brinstar (ブリンスタ Burinsuta) is the default Metroid stage that first appeared in Super Smash Bros. Melee. This stage is based after the first area of Planet Zebes from the original Metroid. The stage consists of a base platform, two sides held by fleshy stalk-like poles, and a hovering metallic platform in the center. There is another fleshy substance in the middle of the base platform that holds the stage together.
The main feature of this stage is the acid which rises from the bottom, which will damage anyone who touches it, and has large knockback (though it can prevent characters from being KOed from the bottom). The acid will periodically rise high enough to cover the entire stage except the top-most platform, forcing players to retreat there to avoid damage. There is also a lone Chozo statue in the background which occasionally gets up and walks around. The fleshy stalk-like poles and substance can be destroyed, the latter of which will separate the bottom platform, breaking the stage in half. Both of the destructible substances will start to regenerate if they are not continuously attacked. While regenerating, the substance that holds the stage together is fall-through. After being destroyed, they will grow back once a certain amount of time has passed. It is considered the best Jigglypuff and Ganondorf stage in Melee (due to rest combos and forward aerial combos with Ganon off the lava) and one of the better Mr. Game and Watch and Wario stages in Brawl (due to air camping beneath the platform).
Brinstar is essentially a redesign of Planet Zebes, from the original Super Smash Bros.
This stage used to be a counterpick stage because many did not see the acid as a major problem, and instead focused on its strange, but relatively non-obtrusive stage design. However, it was later banned in Melee and deemed more of a banned stage than a counterpick stage in Brawl, not making the stage list in major tournaments such as Apex 2012 despite the Unity Ruleset proclaiming it as a counterpick. This stage is still banned because many players agree that it gives monopolizing advantages to certain groups of characters. The acid reduces the stage to a very small size, forcing characters to stay in the air and giving unfair advantages to characters with good aerial games; characters with good meteor smashes or spikes also dominate as they can repeatedly rack up damage by thrusting unfortunate opponents into the acid, which knocks them back up to repeat the process.
Brinstar is the first level that the player has to venture through in original Metroid. In Metroid's Brinstar there are some rooms which feature yellow acid on the ground. If the player goes in the acid it does some damage. Towards the end of Super Metroid, in Mother Brain's room an earthquake occurs and acid starts rising up. While the acid in this stage originated in Metroid, the acid going up and down after an earthquake could be a reference to either Metroid II or Super Metroid (perhaps more likely Super Metroid as Metroid II is set on planet SR388 while Super Metroid is set on Planet Zebes, which is where Brinstar is located).
In Metroid there are secret paths that the player has to take. The secret paths usually involve laying down bombs and exploding certain blocks that led to a new area. In this stage there is this organic material that can be broken up which could likely be a reference to those destructible blocks. Mother Brain's room in both Metroid and Super Metroid feature these things called "Zebetites" which are the life veins for Mother Brain. The player has to shoot them with missiles to destroy them. As the player shoots the veins get thinner and thinner. If the player thins it out and doesn't keep shooting they start to grow back. In this stage two similar veins can be seen. They also retain how the vein can be thinned out and be broken, but in both Metroid games when the veins break they stay broken. This stage has the vein growing back after it has been broken.
Metroid has the player collecting items through the hands of Chozo statues. Super Metroid has kept these Chozo statues except some of them come to life and proceeds to attack the player; these hostile versions are known as Torizos. There are also a few Chozo statues that carry the player across areas that the player wouldn't be able to get to. In the background of this stage, there is a Chozo statue which sits in the same fashion as seen in both Metroid and Super Metroid. There are times when this Chozo statue comes to life and walks across the stage. This is probably a reference to the different types of Chozo statues that come to life in Super Metroid.
In the three Metroid games released at the time of Melee, the player is pretty much always traversing through the caves of planets. This entire stage is featured inside a giant cave. The final boss of Metroid is Mother Brain. As the player shoots her, parts of her pulsate. Mother Brain is also featured in Super Metroid, but she doesn't act in the same manner. In the far background of this stage is a pulsating being which shares some properties with Mother Brain. Out of all the rooms of Brinstar in both Metroid and Super Metroid not one looks similar to the overall design of this stage. 
The track that plays on this stage comes from Metroid. There are three tunes that are mixed together. The first part is the music that plays in Brinstar. The second part comes from when Samus makes her first appearance. The third and last part is the track that plays on Metroid's title screen.