The US space agency's Mars rover Opportunity has for the first time entered a 130m-wide hollow on the Red Planet called Endurance Crater.
The path ahead is littered with round spheres called "blueberries"
The successful foray inside the dip encouraged mission scientists, who are now confident the rover can go deeper.
Scientists want to examine an exposed, dark layer of rock that wraps around the interior of the crater for more clues about water in Mars' past.
But there is still a danger that the rover may get stuck in the depression.
Opportunity rolled in with all six wheels, then back out to the rim to check traction by looking at its own track marks.
The rover is scheduled to drive into the crater for two to three weeks of scientific studies.
The decision to go in was made after much deliberation about the importance of the science targets in Endurance Crater.
The strategy for driving on the crater's inner slope is to keep wheels on rock surfaces instead of sand.
One section of the outcrop mission scientists want to examine is only five to seven metres (16-23ft) from the crater rim in an area dubbed Karatepe.
The rover plan is to get to this part of the outcrop, examine the rocks and then exit the crater.
The team ran trials on Earth with a test rover on a surface specifically built to simulate Karatepe's surface conditions.
The mission team, based at the Jet Propulsion Laboratory in Pasadena, California, decided that the data to be gained from studying the rocks in Endurance was worth sending the rover into the hollow on a one-way trip.
Opportunity has been on Mars since January and has already uncovered past evidence of a body of liquid water at its landing site on the flat plain known as Meridiani Planum.