The bitcoin market has undergone a significant setback with the cryptocurrency's value plummeting to as low as 21,000 US dollars. In recent months, bitcoin had seen unprecedented growth, reaching an all-time high of over 60,000 US dollars. However, the virtual currency saw a sudden downfall, causing concern among investors and experts alike.

There have been several factors contributing to the current state of the bitcoin market. One of the most significant is the regulatory crackdown in China. The country has announced several measures to curb cryptocurrency activity, including banning mining operations and cracking down on trading platforms. This news has had a significant impact on the market, as China is one of the largest bitcoin-trading countries in the world.

Additionally, there have been concerns about the environmental impact of bitcoin mining. The sum of energy consumed by bitcoin mining operations globally has been compared to that of entire countries. Environmental activists have been pushing for regulations and a ban on bitcoin mining to reduce carbon emissions.

Another factor contributing to the decline in bitcoin's value is the overall market sentiment. Many investors have been cautious with their investments lately, given the current uncertainty in the global economy. The ongoing pandemic and the geopolitical environment have created an atmosphere of uncertainty, leading investors to be cautious when investing in high-risk assets such as cryptocurrencies.

Despite the current state of the market, some experts are optimistic about the future of bitcoin. They believe that the regulatory crackdown will eventually lead to a more stable market and improve the overall viability of cryptocurrencies. Additionally, the current dip in the market serves as an opportunity for new investors to enter the market at a lower price point.

In conclusion, the cryptocurrency market, particularly bitcoin, has suffered a significant blow with its value dropping to as low as 21,000 US dollars. The factors contributing to this decline range from regulatory crackdown, environmental concerns, and the overall market sentiment. Nevertheless, some view this as an opportunity for growth and improvement in the market and for new investors to enter at lower prices. The future of bitcoin and the cryptocurrency market, in general, is uncertain, but it's an exciting time for those following the markets.