The U.S. stock market was closed on Monday this week due to the Labor Day holiday. But when it opened on Tuesday major indexes tumbled by more than 1% in a single trading session. The Dow Jones industrial average fell by 234 points, posting its worst day since August 17. Financial stocks saw their steepest decline since earlier this year in May.

One of the issues that spooked the markets was North Korea successfully testing a hydrogen bomb that can be equipped onto an intercontinental ballistic missile (ICBM.) This is essentially a guided ballistic missile with a minimum range of 5,500 kilometers (or 3,400 miles.) Instead of using the missile to deliver nuclear weapons, an ICBM can also be mounted with conventional, chemical, and biological weapons that have varying effectiveness. Most modern designs support multiple independently targetable reentry vehicles (MIRVs), allowing a single missile to carry several warheads, each of which can strike a different target. This latest test was North Korea’s sixth nuclear experiment since 2006, and it’s troubling because it’s the most powerful one yet. As tension between the U.S. and North Korea heats up it understandable that investors are starting to worry about their assets.

The wrath of mother nature is another major concern for U.S. investors. Hurricane Harvey already caused billions of dollars worth of damage to Texas. As oil refineries on the U.S. Gulf Coast start operating again, the oil markets responded with a bullish outlook. U.S. crude prices rose more than 2% on Tuesday, with a 3 week closing high of around $49 per barrel. After making landfall Harvey disrupted about 20 to 25 percent of U.S. refining capacity. But much of that is in recovery this week. Most of the record setting destruction from this storm has already happened. But there’s another hurricane called Irma which is re-raising concerns about the demand for fuel. Irma is a category 5 hurricane, with winds of 160 miles per hour.