The phrase "mass extinction" typically conjures images of the asteroid crash that led to the twilight of the dinosaurs.

Upon impact, that 6-mile-wide space rock caused a tsunami in the Atlantic Ocean, along with earthquakes and landslides up and down what is now the Americas. A heat pulse baked the Earth, and the Tyrannosaurus rex and its compatriots died out, along with 75% of the planet's species.

Although it may not be obvious, another devastating mass extinction event is taking place today — the sixth of its kind in Earth's history. The trend is hitting global fauna on multiple fronts, as hotter oceans, deforestation, and climate change drive animal populations to drop in unprecedented numbers.

These alarming extinction trends are driven by one key factor: humans. According to a 2014 study, current extinction rates are 1,000 times higher than they would be if humans weren't around. A summary of a United Nations report released last month put it another way: "Human actions threaten more species with global extinction now than ever before," the authors wrote.

That report, which assessed the state of our planet's biodiversity, found that up to 1 million plant and animals species face extinction, many within decades, due to human activity.

Read more: Insects are dying off at record rates — an ominous sign we're in the middle of a 6th mass extinction

Other recent research has led to similar conclusions: A 2017 study found that animal species around the world are experiencing a "biological annihilation" and that our current "mass extinction episode has proceeded further than most assume."

Here are 18 signs that the planet is in the midst of a sixth mass extinction, and why people are primarily to blame.