alt=A figure showing that the pressure exerted outside the suction cup exceeds the pressure inside. This pressure difference holds the suction cup in contact with the surface.

Suction cup
Suction cup
Suction cup
The extraction of soil water by the suction-cup method: a review.
Suction cup with solar cell
Development of a semi self-contained wall climbing robot with scanning type suction cups
Sperm whale diving behavior monitored using a suction-cup-attached TDR tag
Twenty-four hourly variations of the aqueous flow; examinations with perilimbal suction cup.
Insights into the Underwater Diving, Feeding, and Calling Behavior of Blue Whales from a Suction-Cup-Attached Video-Imaging Tag (Cri...
Design and suction cup analysis of a wall climbing robot
Development and applications of wall-climbing robots with a single suction cup
Sorption of trace metals by standard and micro suction cups in the absence and presence of dissolved organic carbon.
Suction Cup Sampler Bias in Leaching Characterization of an Undisturbed Field Soil