---
schema_version: "secwatch.filing_event.v1"
accession: "0001628280-26-022818"
form_type: "8-K"
ticker: "WTM"
cik: "0000776867"
company_name: "WHITE MOUNTAINS INSURANCE GROUP LTD"
filed_at: "2026-04-01T23:59:59+00:00"
generated_at: "2026-05-15T08:04:03.303145+00:00"
event_type: "m_and_a"
sentiment: "positive"
materiality_score: 0.6
calibrated_materiality_score: 0.6
confidence: "high"
source: SEC EDGAR
---

# White Mountains acquires majority stake in BaseSix for ~$97M

## Summary
- White Mountains Partners completed acquisition of majority interest in BaseSix Systems for ~$97 million equity investment.
- BaseSix founded 2018, HQ Marietta, GA; provides design, install, retrofit, maintenance of low voltage building systems.
- Company achieved organic growth; plans new offices and strategic acquisitions with WMP backing.
- Co-founders Robb Borden (CEO) and Chris Atwell remain; advisors Morgan Lewis, Deloitte, Miller & Martin.

## SEC filing metadata
- accession: 0001628280-26-022818
- form_type: 8-K
- ticker: WTM
- cik: 0000776867
- company_name: WHITE MOUNTAINS INSURANCE GROUP LTD
- filed_at: 2026-04-01T23:59:59+00:00
- event_type: m_and_a
- sentiment: positive
- materiality_score: 0.6
- calibrated_materiality_score: 0.6
- confidence: high
- sec_items: 8.01, 9.01
- EDGAR index: https://www.sec.gov/Archives/edgar/data/776867/000162828026022818/0001628280-26-022818-index.htm
- EDGAR primary document: https://www.sec.gov/Archives/edgar/data/776867/000162828026022818/wtm-20260401.htm

## Machine-readable alternates
- HTML: https://secwatch.observer/filing/0001628280-26-022818
- JSON: https://secwatch.observer/filing/0001628280-26-022818.json
- Plain text: https://secwatch.observer/filing/0001628280-26-022818.txt

This AI-assisted summary is a reading aid. Review the linked SEC EDGAR filing before relying on any specific claim.