system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's going on with the Chinese stock market plunging?

I've read that the Chinese stock market has plunged by trillions of dollars in the last couple of days.
What is actually happening?
What's causing it?
Is it serious?
gpt
The Chinese economy was built on export, with little consumption.  When the limits of export were reached, it continued to grow by local consumption that consisted almost entirely on investing in production capacity or speculative growth, such as building new cities that did not have enough demand for the housing to fill.   The lack of return on this investment has now limited growth in this area as well.  Much new investment has gone into the stock market, driving it way too high.  Unless China and quickly create consumer demand to replace the speculative investments, the economy is headed for a major slowdown.