The price of iron ore fell below $80 per ton for the first time since September 2009.

The most obvious reason behind the price decline is China, where growth is slowing, which means less industrial production and a deceleration of the country's massive infrastructure build out.

Earlier Tuesday, we learned that the HSBC Flash China Manufacturing PMI climbed to just 50.5 in September. The index remains only marginally above 50, the breakeven level for growth.

"Economic activity in the manufacturing sector showed signs of stabilization in September," HSBC's Hongbin Qu said. "However, overall the data still point to modest expansion. The property downturn remains the biggest downside risk to growth."

What's particularly concerning about the current slowdown is China's reluctance to introduce new stimulus.

"We expect the government to lower its 2015 growth target to 7% [from 7.5%]," Barclays' Jing Chang said in response to the PMI report. "This is in line with the shift in the Premier Li’s attitude – from 'more easing' to 'less stimulus' – that we detected in his comments at the 'Summer Davos' on 9-10 September."

"Uncertainty over the Chinese gov's willingness to add more stimulus are pushing iron prices lower," Bloomberg Chief Economist Michael McDonough tweeted.