The value of bitcoin – the world's most popular digital currency – topped $600 on Monday, as Congress held hearings on the future of this and other currencies that exist only on the internet.

At Mt. Gox – an online exchange where dollars and other government currencies can be traded for bitcoins, and vice versa – the price of the digital currency reached as high as $675, and as of 12:20 p.m. Pacific, it was trading at $640. Bitcoin reached $400 only last week, and at the start of the year, the price was about $13.

Meanwhile, on Capitol Hill, the Senate Committee on Homeland Security and Governmental Affairs is discussing the nature of bitcoin and its role in the Silk Road, an online drug marketplace the feds shutdown earlier this year. The marketplace used bitcoin for payments, as have other illegal operations, but the digital currency also has legitimate uses – many, many legitimate uses.

Various federal agencies said as much to the Committee in letters made public late last week.

Created by an anonymous programmer or group of programmers, bitcoin is a digital currency controlled by thousands of machines spread across the internet – not by a central bank. It allows for anonymous transactions, but at the same time, it keeps a public ledger of payments, so there are ways to crack that anonymity.