Everest has always been a dangerous place, and 2016 has just seen its first deaths of the climbing season. Since last Thursday, Everest has claimed four lives, and two people remain missing.

A 25-year-old named Phurbu Sherpa fell while trying to fix a route just 150 meters below Everest’s summit on Thursday. Eric Arnold of the Netherlands died on Friday while coming back from the summit–a heart attack is suspected. Maria Strydom of Australia died somewhere between Camp IV and the summit on Saturday after a rescue attempt to reach her failed. And Subash Paul died of altitude illness at Base Camp II on Sunday. Two of his teammates have been missing since Saturday are still unaccounted for, according to CNN.

Whenever these first deaths of the season happen–and they inevitably do–it’s easy to say that Everest just isn’t what it used to be. Many of those who perish are a part of the guiding industry–either clients, Western guides or Sherpas–that have overtaken the mountain in recent decades. Some might say that Everest was once was the zenith of exploration, and today it’s a circus of guided groups, frozen bodies and undecomposed human waste. And while it’s true that the experience of climbing Everest has changed dramatically, many of the challenges involved in an ascent remain the same. Here’s the science behind why Everest is such a dangerous and deadly place:

Screen Shot 2016-05-24 at 7.46.41 am

via Forbes