Answer:
They observed the mid-oceanic ridge at the middle of the Atlantic ocean
Step-by-step explanation:
Bruce Heezen and Marie Tharp, both traveled across the large Atlantic ocean in order to create the map of the seafloor. During this time, they observed the presence of valleys and ridges that are present in the middle of the Atlantic ocean. This observation and their study helped in the acceptance of the continental drift theory which was discovered by Alfred Wegener.
This ridge found in the middle of the Atlantic ocean is known as the Mid-Atlantic Ridge, and this is the line along which the seafloor spreading takes place. The rocks that are formed from the melting of magma along this ridge are geologically the youngest ridge.
Thus, the maps that were constructed for the study of the ocean floor revealed the presence of mid-oceanic ridge in the middle of the Atlantic ocean.