Signs just keep on creeping up that the materials sector is in trouble and in today's episode of "As Materials ETFs Tumble," the Market Vectors Steel ETF (NYSE:

) is in the spotlight. On light volume, SLX is down 0.7 percent and while that may not sound like much, Wednesday's decline extends a woeful start to 2013 for SLX and the ETF is now flirting with a year-to-date loss of 18 percent. To put that performance into context, the Materials Select Sector SPDR (NYSE:

),

, is still clinging to a year-to-date gain. SLX's problems are not hard to identify. Just start with the ETF's two largest holdings, Rio Tinto (NYSE:

) and Vale (NYSE:

). Those mining giants combine for over 26 percent of SLX's weight. Rio Tinto's U.S.-listed shares are off almost 24 percent year-to-date and things could get worse before they get better for the Australian mining giant. Not only are Australian exporters being crimped by a strong Australian dollar, but the central bank there expects the mining sector will peak later this year. The Reserve Bank of Australia has already noted that sectors beyond mining and materials must step up to help the world's 12th-largest economy. Then there is Brazil's Vale, the world's largest iron ore producer. Even with today's 2.4 percent pop, Vale's U.S. shares are off 20.5 percent this year. In the case of this stock, investors can debate what is worse: The company's status as the world's leading iron ore maker or the fact that Vale is a Brazilian firm. Brazil has given investors almost nothing to cheer about for over a year now and Vale

. Investors should not just pick on SLX's foreign holdings. In the past month, Nuecor (NYSE:

) is off more than three percent. When accounting for the Wednesday plunges of more than three percent in both Allegheny Technologies (NYSE:

) and Timken (NYSE:

), those stocks are both in the red in the past month. Steel Dynamics (NASDAQ:

) is now about 3.7 percent over the same time. Those four stocks combine for 17.5 percent of the ETF's weight. The hits just keep on coming to SLX. Even before today, U.S. Steel (NYSE:

) was down 12 percent since early March. That stock is now flirting with levels not seen since March 2009. SLX also has the misfortune of being home to Cliffs Natural Resources (NYSE:

), the shares of which have plunged more than 52 percent in the past 90 days. Cliffs and U.S. Steel combine for about 6.8 percent of SLX's weight. Thus far in this piece, approximately half of SLX's weight has been accounted for and none of the news is good. Since it an equity-based ETF is moved by its underlying holdings, it is not surprising to see that

. It appears the ETF is heading for its lowest closing price since late May 2012. Additionally, SLX's ability to defend $40 is vital to any hopes of a rebound because the ETF has not closed below that level in nearly four years. For more on ETFs, click

.