Which mountain was regarded as the highest mountain before Everest was discovered?

 The Chronological list of the Mountains which were once presumed to be the tallest in the world.

• Chimborazo - 6,267 meters (20,561 ft), Ecuador - Until the beginning of the 19th century, this is considered to be the tallest mountain in the world. The tip of the mountain is the farthest from the Earth’s center, because of the equatorial bulge of Earth.

• Nanda Devi - 7,816 meters (25,643 ft), India - Before Kanchenjunga was discovered in 19th century, this is presumed to be the tallest.

• Dhaulagiri - 8,167 meters (26,795 ft), Nepal - From 1808 to 1847, this is considered to be the tallest.

• Kanchenjunga - 8,586 meters (28,169 ft), India - From 1847 until 1852, this is considered to be the tallest.

• K2 - 8,611 meters (28,251 ft), India-Pakistan - Discovered in 1856. This was discovered before Mount Everest was officially declared as the World's tallest Mountain. Considered as the tallest for a very short period.

• Mount Everest - 8,848 meters (29,029 ft). Nepal - Discovered in 1852, but officially confirmed in 1856.

(Images - Google)

Is Mt. Everest the hardest mountain to climb in the world?

 Not according to informed opinions - not mine, but those of the mountaineering community in general.

K2, only 300 feet or so lower than Everest, is more remote, technically more difficult, especially towards the top, has more fickle weather and the routes are subject to more avalanche danger.

K2 has an infamous reputation for killing climbers.

Nanda Devi is almost inaccessible, hidden inside a ring of high mountains with only one narrow way in, through the Rishi Gorge.

Everything has to come in through the gorge. First climbed in 1934 by Bill Tilman and Noel Odell, and an almost modern lightweight expedition, the ascent was not repeated until 1964.

You can see why it attracts climbers, though.

Mount Vinson is the highest point of the Antarctic continent.

It offers little technical difficulty beyond the usual problems of an expedition to Antarctica. Which are considerable.

What is the world's most unusual mountain?

 I will vote for Heart Mountain in Wyoming:

The reason for it being unusual has to do with its composition and how it was formed.

In most places on earth younger rock strata sits on top of older ones. This order is dramatically reversed on Heart mountain. The upper part of the mountain is 350 million to 500 million year old limestone. The lower part of the mountain is 50 million year old sandstone.

how the mountain came to be there is even more unusual. There are no regional mountain building events that can account for it. 50 million years ago the land surface had been largely flat, sloping very gently to the east, and was at a level about half way up what is now heart mountain.

then a colossal land slide occurred about 49 million years ago, when a huge slab of limestone over 1500 feet thick and over 400 square miles in area, that is as thick as tallest building in NYC is tall and as big as the entire city of New York, detached from highland 20 miles to the west, in what is now Yellowstone national park, and slid 30 miles down a gentle 2 degree slope at maximum speed of 60 miles an hour, breaking up into fragments as it went.

heart mountain is one of those fragments.

What is the largest flat top mountain in the world?

 There are many mountains in the world that have a “table top” structure. I will focus on two candidates for the title of the largest flat top mountain in the world.

Mount Roraima: English explorer Sir Walter Raleigh first described this mountain during his expedition to the area in 1596. It is located at a tripoint of three countries: Venezuela/Brazil/Guyana. The mountain’s summit area is defended by 400-metre-tall cliffs on all sides. Its total height is 2,810 m, and its summit surface area is 31km². In short a massive table top mountain.

Here is another view of the top:

Since I posted this answer , there have been many comments about what the top looks like. Here are two photos from World’s Great Wonders by Jheni Osman, that gives readers some inkling what the top looks like:

There is no evidence that pterodactyls or other ancient beast lived on the plateau. Of course the plateau is home to many plant species found nowhere else on Earth. These endemic plants and small animals have developed and are unique to the area.

The second candidate is Table Mountain, Cape Town, South Africa: The table top is 1,086 metres (3,563 ft) above sea level. The length of the “ table” is approximately three kilometres (2 miles). The pic below is the front view as seen from the waterfront area of Cape Town.

Table Mountain is certainly not as high as Mount Roraima, and as far as I can tell from my visits the “flat top” does not exceeds 31 square kilometers. Thus Mount Roraima takes the prize as the largest table top mountain.

But what is clear is that anyone can readily get to the top of Table Mountain via cable car, or you can hike up, which I have done. Here is part of the Smuts trail that starts in Kirstenbosch Gardens that I took:

It takes about 1.5–2hrs to reach the top.

In contrast to get to the top of Mount Roraima requires serious climbing with professionals.

The view from the top of Table Mountain is spectacular. Here is a view from the Top of Table Mountain, with Cape Point in the distance. Note the dam/reservoir on the top of Table Mountain. This pic was taken near Maclear's beacon, the highest point on Table Mountain

Our hike started in Kirstenbosch Gardens following the Smuts trail. Here is a view of Cape Town and Signal Hill from the Top:

And finally a view of Camps Bay on the Atlantic side

So in summary Mount Roraima is the largest flat top mountain the world based on height and the expanse of its table top, but Table Mountain though second in size and height can be readily experienced by all, either by hiking or taking the cable car to the top!

ಭಾರತದ ಪ್ರಮುಖ ಪರ್ವತ, ಕಣಿವೆಗಳ ಮೂಲಕ ಹಾದು ಹೋಗುವ ಮಾರ್ಗಗಳು

Major Mountain Passes in India

ಹೆಸರು •ರಾಜ್ಯ•ಎತ್ತರ (ಅಡಿಗಳಲ್ಲಿ)

1.ಅಸಿರ್ ಘರ್ ಪಾಸ್ •ಮಧ್ಯಪ್ರದೇಶ

2. ಬಾರಾ-ಲಾಚಾ-ಲಾ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •16,400 (ft)

3. ಬನಿಹಾಲ್ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ • 9.291 (ft)

4.ಚಾಂಗ್ಲಾ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •17.800 (ft)

5.ಫೋಟು ಲಾ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •13.451 (ft)

6.ಜಿಲೇಪ ಲಾ ಪಾಸ್ •ಸಿಕ್ಕಿಂ • 14,300 (ft)

7.ಢುಂಬಾರ್ ಕಂದಿ ಪಾಸ್, ಗೋಯೇಚಾ ಲಾ ಪಾಸ್ •ಸಿಕ್ಕಿಂ

8.ಡೊಂಗ್ ಖಲಾ ಪಾಸ್ •ಸಿಕ್ಕಿಂ • 12,000 (ft)

9.ಹಲ್ದಿಘಾಟಿ ಪಾಸ್ •ರಾಜಸ್ಥಾನ

10. ಡೆಬಸಾ ಪಾಸ್ • ಹಿಮಾಚಲ ಪ್ರದೇಶ • 17.520 (ft)

11.ಇಂದ್ರಹಾರ ಪಾಸ್ •ಹಿಮಾಚಲ ಪ್ರದೇಶ •14.473 (ft)

12.ಕುಂಜುಮ್ ಪಾಸ್ • ಹಿಮಾಚಲ ಪ್ರದೇಶ •14.931 (ft)

13.ಖಾರ್ ದುಂಗ್ ಲಾ ಪಾಸ್ •ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •18.380 (ft)

14.ಲಾಮ್ ಖಂಗಾ ಪಾಸ್ •ಹಿಮಾಚಲ ಪ್ರದೇಶ •17.336 (ft)

15.ಲುಂಗಾಲಾಚ್ ಲಾ ಪಾಸ್ •ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •16.600 (ft)

16.ಮಯಾಲಿ ಪಾಸ್, ಮರ್ಸಿಮಿಕ್ ಲಾ ಪಾಸ್ •ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ

17.ನಾಥು ಲಾ ಪಾಸ್ •ಸಿಕ್ಕಿಂ •14.140 (ft)

18.ನಮಿಕಾ ಲಾ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •12.139 (ft)

19.ರೋಹ್ ಟಂಗ್ ಪಾಸ್ •ಹಿಮಾಚಲ ಪ್ರದೇಶ •13.051 (ft)

20.ಪಲಕ್ಕಾಡ್ ಗ್ಯಾಪ್ ಪಾಸ್ •ಕೇರಳ •1,000 (ft)

21.ಸೆಲಾ ಪಾಸ್ • ಅರುಣಾಚಲ ಪ್ರದೇಶ • 14,000 (ft)

22.ಸಸ್ಸೇರ್ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •17.753 (ft)

23.ಟಾಂಗ್ ಲಂಗ್ ಲಾ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ • 17.583 (ft)

24.ಸಿನ್ ಲಾ ಪಾಸ್ •ಉತ್ತರಾಖಂಡ್ •18.028 (ft)

25.ಝೋಜಿಲಾ ಪಾಸ್ • ಜಮ್ಮು ಮತ್ತು ಕಾಶ್ಮೀರ •12.400 (ft)

26.ಟ್ರೈಲ್ ಸ್ ಪಾಸ್ •ಉತ್ತರಾಖಂಡ್ •17.100 (ft)