A surprise eruption of steam in a Yellowstone National Park geyser basin that sent people scrambling for safety as basketball-sized rocks flew overhead has highlighted a little-known hazard that scientists hope to be able to predict someday. The hydrothermal explosion on Tuesday in Biscuit Basin caused no injuries as dozens of people fled down the boardwalk before the wooden walkway was destroyed. The blast came in a park teeming with geysers, hot springs and other hydrothermal features that attract millions of tourists annually. Some, like the famous Old Faithful, erupt like clockwork. But the type of explosion this week is less common and understood, and potentially more hazardous since they happen without warning.