‘At the Mountains of Madness’ is one of my favourite short stories and that is why I really like the film Prometheus. They have the same plot, which I won’t go into because of spoilers.
Here we have Mountains of Madness, the first big-box expansion to Eldritch Horror, which brings the frozen wastes of Antarctica to the globe-trotting adventure game.
This expansion has an add-on board with all the new snowy locations, eight new investigator characters, two new Ancient One enemies, loads more monsters, and what looks like a million new cards.

Cards, cards and more cards. There are new encounters for all the locations, new items, new spells, new conditions, new artifacts, new mysteries for the old ancient ones and all the stuff for the new ones. And other cards I forget.
The new board covers the continent of Antarctica, where the story of ‘At the Mountains of Madness’ is set.
You can travel from the chilly Miskatonic Outpost through the barren Frozen Waste to the deserted City of the Elder Things and finally climb to the legendary and blasphemous Plateau of Leng.

The new add-on board. Good luck finding space on your dining table for this along with all the other stuff you need to play the game.
These locations have neat special actions you can take as an alternative to one of your regular actions, or effects that take place after you resolve an encounter card, so there is a lot more going on here than on the main map.
However, you can’t just slap this board on the table and roam around the icy wastes at will. You only get to use the board if you choose the Elder Things ancient one, or a particular setup option. I guess the effects on the locations are tuned to these adventures. Or maybe it would be a bit of a PITA to have random gates and clues and stuff hidden away on another board. It is awkward to get there, as you might expect.
Even if you don’t use the new board, there are loads of new cards and add-ons to play with on the main board.
The eight new characters have a nice variety of styles and abilities. Some have effects which interact with the new focus tokens, but all of them could be useful in any adventure.
They are: George Barnaby the lawyer, Patrice Hathaway the violinist, Daisy Walker the librarian, Ursula Downs the explorer, Wilson Richards the handyman, Tommy Muldoon the rookie cop, Agnes Baker the waitress, and Finn Edwards the bootlegger.
Way back when, we bought the full set of Arkham Horror investigator miniatures. Luckily they use the same characters in all the AH family of games so we can re-use the figures here.
The two new Ancient Ones are the ice-deity Ithaqua and the polar race of Elder Things.

Notice the small print in the ‘setup’ section of the Elder Things card. You only get to use the Antarctica board if specifically told to.
There are also some additional rules to add new twists to the game.
The most immediately impactful addition is Focus. This is a new kind of token available to investigators. As a regular action, you can get one of these focus tokens. Then you can spend this focus token to re-roll a die during a test. Previously you needed a clue to do a re-roll, but clues are hard to find and usually too much in demand for solving the objectives. Focus gives you precious help in passing tests without holding back your progress.
The other new addition is Prelude cards. These are starting conditions which affect the setup and have a combination of positive and negative effects, and also add a little flavour. The investigators might start with some extra items but there would then be some additional bad stuff waiting for them out in the big wide world. Also, one of these preludes allows you to use the Antarctica board and comes with its own deck of adventures to follow.
The rest of the new cards are simply shuffled into each of the many decks used in the game.
It is hard not to compare Eldritch Horror to its parent, Arkham Horror. I believe the intention of the designers was to take all that was good in Arkham Horror, polish it, and take it to a global scale.
I think they totally succeeded and did so while maintaining more or less the same level of complexity. If you compare the ‘feel’ of the two games, Arkham Horror feels claustrophobic while Eldritch Horror feels more wide open. In Arkham Horror you are trapped in a small town with weird scary stuff closing in on you, but in Eldritch Horror you are zooming around the world from exotic location to exotic location having adventures along the way. Both are fun.
Personally, I think it is the different way that monsters work in the two games that has the biggest effect on their ‘feel’. In Arkham Horror, monsters wander around the town and you have to evade or fight them if you want get by. In Eldritch Horror you can largely ignore a monster unless you need to do something at that location, and this gives you more freedom of movement.
Having said that, Arkham Horror did feel much ‘richer’ than Eldritch Horror, maybe because of all its expansions. After two or three plays of core Eldritch Horror it kind of feels the same, even with different Ancient Ones and investigators.
I think the key is the location cards. These provide the story-telling aspect of the game and you need a decent variety of these to keep it fresh.
Adding more cards through this expansion and its new map and locations provides the huge boost the game needs to give it that extra replayability. (Plus you have two smaller expansions available with additional ancient ones and cards.)
Recently, Fantasy Flight Games announced the next big-box expansion, Under the Pyramids. This will have more investigators, more ancient ones and another new board. I don’t think it will be long before Eldritch Horror will rival Arkham Horror in sheer scope and variety.