Bitcoin awoke from its slumber this week. The cryptocurrency’s price, which after reaching $20,000 in 2017 has languished near $4,000 for most of 2019, surged more than 20% on Tuesday (April 2), surpassing $5,000. The cause? A mystery trader—or group of traders—reportedly placed a series of large orders on major exchanges, including Coinbase, Kraken, and Bitstamp.

“There has been a single order that has been algorithmically-managed across these three venues, of around 20,000 [bitcoins],” said Oliver von Landsberg-Sadie, founder of BCB, a cryptocurrency financial services firm. “If you look at the volumes on each of those three exchanges – there were in-concert, synchronized, units of volume of around 7,000 [bitcoins] in an hour,” he explained to Reuters.

Indeed, order books reflect the surge, which occurred between 9:30 pm and 10:00 pm PT April 1. Here’s the view from Kraken: