As we all are familiar with the Blockchain now, we know that it is immutable. No member in blockchain can alter it in any way. But what if, not one member, but some group of members try to alter blockchain? What if these members are in the majority? Will the system still remain immutable? That's where your 51% Attack comes into the picture. Let's have a look at this scenario in detail.

What is 51% Attack?
51% attack is an attack on blockchain performed by some gatherings of miners. Basically, they control more than 50% of the network's computing power. The attackers then can manipulate the transactions. They can disallow new transactions to get confirmation. They may also be able to reverse some transactions that which were done when they were in control of the network.

This attack occurs when blockchain is not fairly distributed across multiple independent nodes and miners. It becomes very easy for miners to launch a 51% attack because they will be very few in numbers. If any blockchain network is fairly distributed then there is a good chance that it is not prone to 51% attack. It becomes next to impossible to perform a 51% attack.

Why it's risky?
Although 51% attack does not produce any cryptocurrency or they cannot affect the blockchain directly, but they still can surely reduce the faith of participants which they have in cryptocurrency. If any transaction gets blocked then users will face discomfort and start panicking, which is not good for a company's reputation.

Some well-known examples of 51% attacks are as follows:

Krypton and Shift
They both are blockchains based on ethereum. They have suffered 51% attacks in August 2016.

Bitcoin Gold
Bitcoin Gold suffered a 51% attack in May 2018. A vast amount of Bitcoin Gold's hash power was controlled. The attackers were successfully able to double-spend for several days. They were able to grab more than $18 million worth of Bitcoin Gold.

ZenCash
ZenCash also suffered from 51% attack in May 2017. Attackers were successful in double-spending an overall of two huge transactions worth 13,000 ZEN and 6,600 ZEN.

Conclusion
Is very difficult for an attacker to get more computational power than the remainder of the network. On the other hand, it becomes easy to go for smaller cryptocurrencies. At the point when contrasted with Bitcoin, altcoins have a moderately low measure of hashing power verifying their blockchain. The power is low enough to make it feasible for a 51% attack to really occur.