Over Sea, Under Stone, chapter 10

In which the kids hang out at at Kemare Head, and make an important discovery.

Well, first they investigate the rocks revealed by moonlight, and Barney volunteers that (according to Mr. Penhallow from chapter 1) the locals avoid the area, calling it the "gravestones."  This gets him thinking of Sir Bedwin being buried "over sea and under stone", and they search the rocky headland for any possible place the grail could be buried--skeptical Simon points out that the area's mostly bedrock with very little soil, so it's unlikely anything is buried there, but since they're out of ideas, Jane suggests methodically searching for a place the grail could be buried.

They don't find it.  Jane does pick up a neat petrified shell, however.

Everyone's getting pretty morose about never finding the grail at this point, when, fortunately, Rufus the dog gets lost.  In a transparent Deus ex Canine, they find him pawing at one of the rocks, which Simon discovers has a tiny gap in it.  He and Jane manage to shift it, revealing a seemingly bottomless hole.  "This must be it, mustn't it?  It must be where he hid the grail!"

Another clever idea for Jane: dropping a rock into the hole and listening for it to hit the ground. Unfortunately, it's too deep for them to hear anything.  We get another episode of 'fun details from their outside lives' when the kids turn out their pockets looking for something that will help them measure the depth of the gap.  [Simon loses another five points from Gryffindor for the "Two hair-grips. Just like a girl" remark.  Yes, Jane wears her hair long enough that having spare hair ties is useful.  It's not an inherent trait of her gender, nor does it require commentary from you, Simon.] Between the three of them, they work out a system using Jane's spool of thread, a pencil and Barney's scrap of wire, which determines that the hole is over 150 feet deep.

This knowledge probably won't help then get the grail, but while determining it, Jane noticed that she could hear the sea through the hole in the headland: it's the vent to a cave.  A cave with some sort of seaward outlet.

There's a Weathertop Moment, when Simon sees the Lady Mary (the Withers' yacht) and everyone hunkers down out of side behind the rocks.  Suddenly, they realize it's 11:30 and Great-Uncle Merry hasn't found them yet.  Could he still be sleeping?

My favorite bit of this chapter is the how the kids go about solving their problems creatively (new headcanon: Simon grows up to be less sexist, Jane gains some more confidence, and they both become engineers).  Least favorite bit is how often decisions are made based on 'having a feeling' rather than solid evidence: the grail must be hidden under these rocks, it must not have ended up underwater and degraded, time is running out, etc.  It does allow complications to be brought up, but it also means they are being dismissed rather than addressed.  Nonetheless, the race is clearly on!