Investigation (3)

"It might not be on the blueprints," Hermione said. "Some entrances were sealed after the blueprints were drawn. People later on might not have known they existed."

Delaney stood up, switched off the detector, put it back into the case, and turned around. "We'll stop here tonight. Next week, I'll bring a set of more precise ranging equipment. It can map out the complete contours of the space beneath that crack. If there really is a buried room, the equipment can determine its dimensions and the location of its entrance."

The four of them headed up the stairs. The return journey was quieter than when they had come, as though part of their attention had yet to move away from that crack and the feel of the flat structure beneath it.

After the lock had been put back on, Delaney stood in the corridor for a moment. "I can get the ranging equipment on Wednesday night. If necessary, we can come again on Thursday night."

"Eleven on Thursday?" Harry asked.

"Eleven." Delaney turned and left.

On Thursday night, the fourth-floor corridor was still quiet.

Delaney was already more practiced at picking the lock than last time. The latch sprang open with a short, crisp sound, like a rhythm repeated too many times.

He hung the lock aside, pushed open the door, and only started walking after the others had caught up.

This time, he had brought a larger metal case, roughly two feet square, with two reinforcing ribs on its surface and a leather handle.

He opened the case on the floor, took out several pieces of equipment, and connected them together with practiced ease.

One of the probes was thicker than the last one, its tip conical, as though it integrated several sensing surfaces.

He inserted the probe into