The world’s largest Bitcoin exchange Mt. Gox is being hit by a distributed denial of service (DDoS) attack. The site is currently down, just over two hours after it resumed trading earlier on Thursday night.

In a post on Google+, the exchange merely says “We are experiencing a stronger than usual DDoS. We are working in (SIC) it.”

Indeed, a quick visit to mtgox.com shows that the service is indeed unavailable:

mtgox down 730x596 Hours after reopening trading, top Bitcoin exchange Mt. Gox taken down by stronger than usual DDoS attack

Over at Bitcoinity, we can see that the last trade on Mt. Gox was for $109.26. Earlier today, the company temporarily halted its exchange services, causing Bitcoin to plummet. Ironically, the move was made in response to a system overload yesterday, which also significantly devalued the virtual currency.

For those who haven’t been following, here’s the timeline of events. Bitcoin started to crash on Wednesday. Mt. Gox was having problems dealing with the load, but assured all that “no we were not last night victim of a DDoS but instead victim of our own success!”

Here’s what happened, according to Mt. Gox:

“Indeed the rather astonishing amount of new account opened in the last few days added to the existing one plus the number of trade made a huge impact on the overall system that started to lag. As expected in such situation people started to panic, started to sell Bitcoin in mass (Panic Sale) resulting in an increase of trade that ultimately froze the trade engine!”

On Thursday, the company posted an announcement titled “Market Cooldown for 12 hours”:

“Orders will not be accepted for the moment as we need to upgrade our database to accommodate the trading volume. However, you may still cancel your pending and open orders. Trading will resume at 11.00 am JST. Our apologies for the inconvenience caused and thank you for your patience while we work to resolve this issue.”

Unsurprisingly, because of Mt. Gox’s size, this sent Bitcoin hurtling downwards. Bitcoin went from a high of $266 on Wednesday (its highest yet) before tumbling down to $150, and then hitting a low of about $130. After Mt. Gox halted trading, it was at around $65.

It started to recover, however, but when Mt. Gox reopened, it crashed again, settling in around the $75 mark.

Depending on which exchange you trust, here’s the current situation (numbers courtesy of Bitcoin Watch):

btceUSD – $73.9900
bitstampUSD – $67.8100
bitfloorUSD – $85.0000
cbxUSD – $94.0000
btc24USD – $60.0000
vcxUSD – $86.0000
rockUSD – $70.0000
icbitUSD – $85.0000

Bitcoin is very unstable as it is. With Mt. Gox’s going in and out, however, there’s really no telling where it will end up.