The first four months of 2022 have been challenging for the cryptocurrency industry. Bitcoin has struggled around the $40,000 level. It's therefore not surprising that Bitcoin mining stocks have also trended lower. For investors who are bullish on Bitcoin, the correction provides a good opportunity for exposure. Among Bitcoin miners, Riot Blockchain (RIOT) seems to be attractively valued. RIOT stock has plunged by almost 75% in the last 12 months. Besides the downtrend in Bitcoin, another key reason for the sharp sell-off is equity dilution. However, I believe that RIOT stock looks undervalued at current levels of $10.40.