China's main share index, the Shanghai Composite Index, is trading 50% below the peak level reached in October 2007.
It ended the morning session at 3,022.6, which is 3% down on the day and less than half of its record close of 6,092.1, set on 16 October.
Concerns that government measures to control inflation could hit the profits of Chinese companies have been partly behind the drop.
The index had previously risen almost sixfold in two years.
The government has been taking steps to support share prices.
On Sunday, the China Securities Regulatory Commission announced restrictions on sales of certain shares.
Further measures are expected, including a possible reduction in the tax on share trading if the market keeps falling.
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