In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Q: Passage: Ice drilling allows scientists studying glaciers and ice sheets to gain access to what is beneath the ice, to take measurements along the interior of the ice, and to retrieve samples.  Instruments can be placed in the drilled holes to record temperature, pressure, speed, direction of movement, and for other scientific research, such as neutrino detection.
Many different methods have been used since 1840, when the first scientific ice drilling expedition attempted to drill through the Unteraargletscher in the Alps.  Two early methods were percussion, in which the ice is fractured and pulverized, and rotary drilling, a method often used in mineral exploration for rock drilling.  In the 1940s, thermal drills began to be used; these drills melt the ice by heating the drill.  Drills that use jets of hot water or steam to bore through ice soon followed.  A growing interest in ice cores, used for palaeoclimatological research, led to ice coring drills being developed in the 1950s and 1960s, and there are now many different coring drills in use.  For obtaining ice cores from deep holes, most investigators use cable-suspended electromechanical drills, which use an armoured cable to carry electrical power to a mechanical drill at the bottom of the borehole.
In 1966, a US team successfully drilled through the Greenland ice sheet at Camp Century, at a depth of 1,387 metres (4,551 ft).  Since then many other groups have succeeded in reaching bedrock through the two largest ice sheets, in Greenland and Antarctica.  Recent projects have focused on finding drilling locations that will give scientists access to very old undisturbed ice at the bottom of the borehole, since an undisturbed stratigraphic sequence is required to accurately date the information obtained from the ice.
A:
Percussion and rotary drilling are two early methods of what?