Word to describe "when someone describes something in too much detail"

There's a word I thought I knew at some point, but can no longer remember what it was.

I tried looking up various thesaurus websites to no avail.

Similar words to what I'm looking for, but not quite:

• Elaborate: too neutral -- I'm looking for a more negative connotation.
• Elucidate: too positive a connotation.
• Ruminate: The direction is correct, but it is about "thinking about something in too much detail" vs actually describing it so.
• Describe ad nauseum: 3 words, and awkward sounding.

Example sentence: "He began to __ the topic"

To clarify, I'm looking for a verb.

It could be expatiate:

from the OED:

To speak or write at some length; to enlarge; to be copious in description or discussion.

from Merriam-Webster:

to speak or write at length or in detail

"Verbose" can be used for for that purpose, meaning overly wordiness, in general.

From Merriam-Webster Online Dictionary:

Main Entry: ver·bose

Pronunciation: (ˌ)vər-ˈbōs\ Function: adjective Etymology: Latin verbosus, from verbum Date: 1672 1 : containing more words than necessary : wordy ; also : impaired by wordiness 2 : given to wordiness synonyms see wordy — ver·bose·ly adverb — ver·bose·ness noun — ver·bos·i·ty -ˈbä-sə-tē\ noun

You could say that they were waxing prolix, which sounds a lot nicer than that they were prattling on, but serves the same purpose.

EDIT 24 April 2016

He began to belabor (US) / belabour the topic

belabour
2. Argue or discuss (a subject) in excessive detail:
- there is no need to belabour the point
- This is especially the case when those words simply amount to belabouring the obvious.

Explicate verb,

To explain meticulously or in great detail; to elucidate; to analyze.

-The real difference is that Auerbach is attempting to explicate the text in front of him, to help the reader "see" more fully what is really going on in the scene from the Odyssey.

He began to explicate on the topic