We know that the Taj Mahal is one of the seven wonders of the world. We know who built it and for whom. We know it is the pride of our nation, India. We know a lot about it. But we don't know a lot of things about it.
Let's look at some of them:

1. There is a small hole on the ceiling of the main hall perpendicularly above the tombstone of Mumtaz Mahal .

According to stories it is the work of an artisan who decided to sabotage Shah Jehan 's dream of creating a masterpiece after he came to know about his decision to amputate all the artisan's arms. So the Taj is not as flawless as we would like to think it is.
Source: Wallpaper UP

2. The ASI had to conceal the Taj during WW-II  with a huge scaffold to misguide bombers.

One of the most majestic man-made creations, the Taj was covered with a huge scaffold in the 20th Century so that it looked like a stockpile of bamboo to bombers. Then d uring the India- Pakistan war in 1971, and after 9/11, it was protected by camouflaging it with a green cloth.
Source: vustudents

3. Ever wondered what was Shah Jehan's first reaction on beholding the great Taj?

"Should guilty seek asylum here,Like one pardoned, he becomes free from sin.Should a sinner make his way to this mansion,All his past sins are to be washed away.The sight of this mansion creates sorrowing sighs;And the sun and the moon shed tears from their eyes.In this world this edifice has been made;To display thereby the creator's glory."
Source: I times

4. The story of the arms of all artisans being amputated by is probably a myth because the architect worked on other projects.

The architect who lead the team of architects to build the Taj mahal laid the foundation of the Red fort too. Ustad Ahmed Lahauri, was probably the leader of the architect's team. Seeing that Shah Jehan commissioned him to work on a project after Taj means he was clearly using both his arms!
Source: Tebyan

5. The minarets were built tilting outwards to protect the Taj from calamities like earthquakes.

If you observe carefully, you'll find that the four minarets are tilting outwards. Thi s was done so that in the event of a natural disaster, like earthquake, the minars won't fall on the main ' gumbad ' or Dome, thereby ensuring the safety of the Tomb.
Source: Nitish Bhardwaj

6. A temple was built for the conman Natwarlal, who sold the Taj Mahal repeatedly.

Natwarlal who repeatedly sold the Taj Mahal , has a temple on his name. The people of his native village Bangra in Bihar decided to put up a statue of him as his monument, at the place, where his house once stood.
Source: Travel dictionary

7. The foundation of Taj Mahal would have eroded years ago if Yamuna wasn't there.

Taj's foundation is made of timber which is not supposed to be long lasting. T he wood should weaken overtime and crumble owing to rot and ruin, but that did not happen because the wood is kept strong and moist by the Yamuna river.
Source: Wikipedia

8. The interior of Taj is capable of blinding any goblin with its grand inlay work.

28 kinds of rare, semi precious and precious stones were used for inlay work in the Taj Mahal . They were sourced from Sri Lanka, Tibet, China and of course several places in India. During the British rule, the monument was violated several times because of these stones and it was only in the late nineteenth century that restoration work was taken over.
Source: Islamic finder

9. The Taj Mahal is taller than Qutub minar.

The Taj Mahal is about five feet taller than Qutub Minar. A comparison with other monuments around the world can be found below.
Source: namhatta

10. How expensive was it for Shah Jehan to build the Taj exactly?

Shah Jehan spent about 32 million rupees during the years 1632-1653 on this great architectural venture. Today that amount would be close to 1,062,834,098 USD.
Source: Topteny

11. The fountains have a special feature to ensure uniform water pressure in the fountains.

To ensure uniform and undiminished water pressure in the fountains, the fountain pipes were not connected directly with the copper pipes feeding them. Instead a copper pot was provided under each fountain pipe so that the water first fills the pot and then only rises at the same time in the fountains. The pressure in the pots is maintained and so in the fountains.
Source: Tracy and Dale

12. PC Sorkar Jr. made the Taj Mahal vanish in the year 2000.

PC Sorkar Jr. managed to create an optical illusion that made the Taj vanish from the eyes of the public at Kachhpura in Agra on 8th November 2000.
Source: Thought unbound

13. Around 12,000 visitors come to see The Taj every day.

One of the seven wonders of the world, the Taj sees many visitors everyday.
Source: Bopworld

14. Shah Jehan wanted to build another Taj Mahal , a black Taj Mahal .

Emperor Shah Jehan is widely believed to have desired a mausoleum for himself similar to that of the one he had built in memory of his wife, Mumtaz Mahal . The writings of the European traveller, Tavernier mention that Shah Jehan began to build his own tomb on the other side of the river but could not complete it as he was deposed by his own son Aurangzeb .
Source: Krisandr

15. George Harrison clicked a selfie at the Taj .

One of the first selfies taken at Taj were by George Harrison. He used the fish-eye lens.
Source: Electric

16. T he plinth of the Taj varies all around.

The plinth of the tomb is 2'10" on an average but varies all around. T his convexity has deliberately been given to the plinth in the center of each arch. The architect fully anticipated the apparent size which a finial would present from such a great height. If this wasn't the way then the building would have appeared as if it were falling down.
Source: Imgkid

17. The Taj changes colors depending on the light.

The Taj is pinkish in the morning, milky white in the evening and golden when the moon shines.
Source: Cntraveller

18. Shah Jehan's daughter Jahanara Begum refused a grand burial like the Taj .

She said:
Allah is the Living, the Sustaining. Let no one cover my grave except with greenery, For this very grass suffices as a tomb cover for the poor. The mortal simplistic Princess Jahanara, Disciple of the Khwaja Moin-ud-Din Chishti, Daughter of Shah Jahan the Conqueror May Allah illuminate his proof.
Source: Times city

19. An American Grammy Award-winning artist named himself Taj mahal.

Henry Saint Clair Fredericks, who uses the stage name Taj Mahal, is an American Grammy Award-winning Blues musician.
Source: chancentre

20. The makers employed an optical trick so that as you move closer to the gate, the Taj keeps getting smaller.

It seems to grow bigger as you walk away. The guides here say when you leave,you take the Taj with you in your heart.
Source: Travelblog