You need to collect the 10 items from the Lost Expedition page in the Museum. All of the items are randomly spread throughout the level (though many are found deeper in the ground).
The achievement will unlock as soon as you pick up the final item. I unlocked this during the campaign play of the Antarctic level, collecting all the items in one play through. If for some reason you miss one in campaign play, start a new game in Freeplay mode since your digger is fully upgraded from the beginning and you can drive deep into the map quicker to find your missing item.