  MACS J0600.1-2008 (MACS0600) is an X-ray luminous, massive galaxy cluster at
$z_{\mathrm{d}}=0.43$, studied previously as part of the REionization LensIng
Cluster Survey (RELICS) and ALMA Lensing Cluster Survey (ALCS) projects which
revealed a complex, bimodal mass distribution and an intriguing high-redshift
object behind it. Here, we report on the results of an extended strong-lensing
(SL) analysis of this system. Using new JWST and ground-based Gemini-N and Keck
data, we obtain 13 new spectroscopic redshifts of multiply imaged galaxies and
identify 12 new photometric multiple-image systems and candidates, including
two multiply imaged $z\sim7$ objects. Taking advantage of the larger areal
coverage, our analysis reveals a new bimodal, massive SL structure adjacent to
the cluster which we measure spectroscopically to lie at the same redshift and
whose existence was implied by previous SL-modeling analyses. While based in
part on photometric systems identified in ground-based imaging requiring
further verification, our extended SL model suggests that the cluster may have
the second-largest critical area and effective Einstein radius observed to
date, $A_{\mathrm{crit}}\simeq2.16\,\mathrm{arcmin}^2$ and
$\theta_{\mathrm{E}}=49.7''\pm5.0''$ for a source at $z_{\mathrm{s}}=2$,
enclosing a total mass of
$M(<\theta_{\mathrm{E}})=(4.7\pm0.7)\times10^{14}\,\mathrm{M}_{\odot}$. Yet
another, probably related massive cluster structure, discovered in X-rays $5'$
(1.7 Mpc) further north, suggests that MACS0600 is in fact part of an even
larger filamentary structure. This discovery adds to several recent detections
of massive structures around SL galaxy clusters and establishes MACS0600 as a
prime target for future high-redshift surveys with JWST.
