/u/The_Alaskan made an excellent post summing up the historical discussion around the sinking of the Maine here. In it, they summarise the evidence for and against the competing theories. While there is a not inconsiderable amount of evidence that suggests that Maine might have been sunk by a mine, I consider it more likely that she was sunk by an internal explosion. Coal bunker fires were repeated and frequent issues with contemporary ships. Similarly, accidental magazine explosions were not infrequent aboard warships, with the RN, French and Japanese navies all experiencing them in the years leading up to WWI. Additionally, there was little evidence of an explosion outside of Maine's hull - no spout of water or dead fish on the surface, both signs of an underwater explosion.
I found a rather good article on the subject here, though it is unfortunately paywalled. If you personally work/study at a university or have friends or family who do, they may be able to download it for you.
As /u/The_Alaskan 's post offers a more technical explanation, here is a bit more of a summarized narrative version.
There does not seem to be an 'open and shut case' for any one explanation of the USS Maine's destruction, but the evidence largely points to it being an accident. Let's walk through the different possibilities and see why they're more or less likely.
Despite so many contemporary records being open to the public, we have yet to find a single shred of evidence that this was either a black flag operation by the US, part of a Spanish plot to destroy the ship, or an act by Cuban guerrillas fighting for independence who wanted the US to intervene. Absence of evidence is not evidence of absence (a point I'll make more than once here), but as of this moment we still have no proof, so accusations of it being intentional are entirely speculative.
Here is where most of the disagreement comes from. There are two main explanations for what caused the Maine's destruction. The earliest is an underwater mine of some kind detonating the ship's munitions stores. The second, and currently the more accepted one, is that it was likely an internal explosion.
As Alaskan already points out, four investigations took place. The first two (1898, 1911) disagreed on the details, but agreed as to the principal cause of the explosion; an underwater detonation from some kind of mine. Half a century later, in 1975, Admiral Rickover headed a new investigation which suggested an internal explosion. A final investigation was done at the behest of National Geographic, in 1997, and published in its February 1998 issue, which suggests the issue is still a mystery.
The article I linked to at the beginning makes a very solid case for it being near impossible for it to have been a mine.
First, experts weighing in on the forensic evidence from the wreck found that the damage was consistent with an internal detonation and showed no signs of mine damage. Things that were inconsistent with the 'internal explosion' hypothesis, like some parts of the ship being dented inwards instead of outwards, were caused (in the opinion of the 1976 study and the article's author's own research) by explosion physics.
Specifically, the thermal reaction pressed out in all directions. As the explosion expanded in all directions, the direction of least resistance ended up being upwards (air, therefore less pressure) as opposed to sideways or downwards (water, hence more pressure). This is all happening really, really quickly, mind you. When all the oxygen gets used up by the explosion, this causes a sudden drop in pressure inside of the ship, while water pressure outside of the ship stayed the same. The water pressure then pushed some parts of the ship (which was simultaneously being ripped apart by the explosion) to bend inwards.
Try this experiment. Get an empty soda can and pour about 1/4th of its space up with water. Heat the can over a stove or some other heat source (be careful, don't play with fire lightly, use safe metal tools to grab and handle this). Now grab a large metal bowl and fill it with cold water. The colder, the better. Once the water in the can is boiling, use metal tools of some kind to grab the can and, quick as you can, flip it onto the bowl with cold water. Specifically, make sure all the hot water stays inside the can as you do this and make sure you flip the can upside down, so that the opening is in contact with the cold water. What happens to the can? It implodes. That's roughly what happened to the ship, but because this was part of a larger process, the 'can' (ship) wasn't perfectly sealed, etc., the can you use will be much more deformed than the ship was.
Now, why else can't a mine work? For one thing, no water plume in the water was seen and only one explosion was heard (the munitions stores).
In addition, how do you plant the damn thing without getting noticed? A mine that could destroy the Maine (an armored ship which was only 3 years old in 1898) would have been enormous. To get it to the Maine you'd have needed a small ship, which would have drawn attention. The alternative would have been to chain it to the bottom of the bay and wait for the Maine to come in. Something which suggests a huge degree of premeditation, but would be even more difficult. How does a team of soldiers plant an enormous aquatic mine in the middle of a well trafficked harbor without grabbing anyone else's attention or accidentally destroying someone else's ship? The Maine itself was, in addition, actually arriving earlier than expected, so if it was the target, the conspirators would have needed to know exactly where it was going, have needed to plant the enormous mine in a very public area with no one noticing, and have known that the ship was arriving early.
As you can see, every step you take towards the mine theory requires you to make several more leaps of faith in order for things to make sense.
All this for a mine which we have no evidence ever existed in a plot for which we have found no evidence.
Basically.
Worse, it doesn't make any damn sense for one to be there either.
A mine of that size, hitting at that exact spot, would have (according to the experts cited in the article I linked to) either needed to have been taken there on a smaller ship or chained at exactly the right place and height. That implies a conspiracy.
So, qui bono? Who benefits?
The Spanish had no reason to destroy the ship, as they already had their hands full with rebellion in Cuba and other colonies, and were not ready for a global naval war with the Americans. And, again, no paper trail.
The Cubans could have, theoretically, wanted American help fighting the Spanish, but we have absolutely no evidence they planned this and accusing them implies that in the highly militarized capital of the colony (Havana) a team of crack Cuban explosives experts were able to steal through the night like some kind of elite military infiltration team, plan an enormous mine, and get out without leaving a trace. If the Spanish had even had a whiff of this plot they'd have every reason to have shut it down or publicized it. So, no dice.
The Americans? #MckinleyDid02/15 conspiracy theories aside, we have 0 evidence that this happened. Most of the objections to the Cubans being behind it also apply. The leaps of scope of the operation required to believe in a conspiracy like this make it unlikely in the extreme.
Evidence points to the munitions stores being set off. What set them off? One explanation is that the coal furnace (steam powered coal fueled vessel, remember) which was near the munitions stores heated them to the point that they went off. There were barriers between the furnace and the munitions meant to stop this kind of accident, but the same kind of design flaw had claimed similar vessels in the past and is consistent with the evidence we have.