The Russian ruble just sank to the lowest point in its history, above 50 to the US dollar.

It's down 5.27% against the US currency Monday alone. The dollar is about 50% stronger against the ruble than it was at the start of 2014. This follows the crumbling price of oil.

Here's what the US dollar-Russian ruble exchange rate looks like over the past five years:

The fall in oil prices has dramatically weakened the Russian currency, which had been much more stable in the previous three years.

Brent crude is down to just $68, and the other major benchmark of oil prices, West Texas Intermediate (WTI), is down to just $65, a five-and-a-half-year low.