There's a mole on Mars that's making NASA engineers tear their hair out.

No, they haven't discovered a small, insectivorous mammal on the red planet.

The mole vexing engineers at the Jet Propulsion Laboratory in Pasadena is a scientific instrument known as the Heat Flow and Physical Properties Package, or HP3 — or just "the mole" — carried on NASA's InSight probe that landed on Mars a year ago.

"The mole is designed to measure heat flow coming out of the interior of Mars," says Troy Hudson, InSight's instrument system engineer.

Scientists are interested to know how much heat is still being generated inside the core of the once geologically active Mars. To do that, the mole has to bury itself about 16 feet below the Martian surface so it won't be affected by daily temperature fluctuations.

The mole is basically a tube about 16 inches long and an inch in diameter. It has a pointy tip and an internal hammer that works like a kind of pile driver to pound the instrument into the ground.

The frustrations began last February when the digging started. Instead of going down to 16 feet, it got stuck after just 14 inches.

Hudson says he and his team decided the problem was related to bouncing.

Just like a gun recoils when you fire it, the mole recoiled ever so slightly every time the hammer tried to drive it into the ground. So instead of going down, it bounced in place.

Some of the problem may be caused by uncertainty about the size of the grains of Martian sand. It turns out that pushing a probe into something like flour is very different than pushing it into a bowl of granulated sugar.

The low pressure on Mars also changes the way the dirt behaves compared to soil on Earth.Engineers thought they might be able to prevent the bouncing if they used the scoop on InSight's robotic arm to press against the mole as it hammered. They tried that a few weeks ago — and it worked.

"For the first time in 8 months we have definite forward progress," Hudson told NPR at the time.

But Hudson knew there was another looming problem. Eventually, the top of the mole would be flush with the Martian surface, and there would be nothing for the scoop to press against.

So they came up with a new plan.

"We moved the scoop over to a different position nearby, and pushed hard on the soil, hoping that would transfer force to the mole through the soil rather than directly," he says.They sent instructions for the scoop to press, the mole to hammer, and for Insight's camera to record what happened.

Hudson says he was horrified when he saw the pictures.

"I was very distraught," he recalls.

The mole had backed almost half-way out of the hole, inadvertently undoing much of their progress.

Hudson is pretty sure he knows what happened. Without the scoop pressing on the mole, it started bouncing again.

"When it does that, loose soil in front of the mole can infiltrate in front of the tip, filling up the space that occurs whenever it bounces," Hudson says. "Then it's just bounce, bounce, bounce, bounce, and more material fills in and it ends up backing out of the ground."

Hudson says he and his team are confident they can use the scoop press technique to get the mole back down to where it was. They've already made an inch and a half of progress.

Once the top of the Mole is again flush with the surface, he says "we're going to have to come up with a new way to get it underground fully and we haven't figured out exactly what we're going to do there, yet."