Filling the gaps

Room after vibrant room, the Doctor was getting gloomier. He had carefully coloured every piece of every map. Each corner of Gallifrey, Tara, the planets of E-space - he had already visited every interesting landmark (and most of the uninteresting ones) throughout the multiverse.
Downhearted, he walked back, trailed by his scarf.
By accident, he glimpsed a narrow white strip on his favourite planet.
***
Romana looked at the dark scanner: "It's broken. Again."
"No," the Doctor beamed. "It's the Mariana Trench. I've never been here."
Romana sighed: "Well, as untarnished blacknesses go, it's first rate."
"Yes," he said blissfully. "It is."